|Classless Inter-domain Routing (CIDR): The Internet Address
Assignment and Aggregation Plan
|V. Fuller, T. Li
Network Working Group V. Fuller
Request for Comments: 4632 Cisco Systems
BCP: 122 T. Li
Obsoletes: 1519 Tropos Networks
Category: Best Current Practice August 2006
Classless Inter-domain Routing (CIDR):
The Internet Address Assignment and Aggregation Plan
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
This memo discusses the strategy for address assignment of the
existing 32-bit IPv4 address space with a view toward conserving the
address space and limiting the growth rate of global routing state.
This document obsoletes the original Classless Inter-domain Routing
(CIDR) spec in RFC 1519, with changes made both to clarify the
concepts it introduced and, after more than twelve years, to update
the Internet community on the results of deploying the technology
This memo discusses the strategy for address assignment of the
existing 32-bit IPv4 address space with a view toward conserving the
address space and limiting the growth rate of global routing state.
This document obsoletes the original CIDR spec [RFC1519], with
changes made both to clarify the concepts it introduced and, after
more than twelve years, to update the Internet community on the
results of deploying the technology described.
2. History and Problem Description
What is now known as the Internet started as a research project in
the 1970s to design and develop a set of protocols that could be used
with many different network technologies to provide a seamless, end-
to-end facility for interconnecting a diverse set of end systems.
When it was determined how the 32-bit address space would be used,
certain assumptions were made about the number of organizations to be
connected, the number of end systems per organization, and total
number of end systems on the network. The end result was the
establishment (see [RFC791]) of three classes of networks: Class A
(most significant address bits '00'), with 128 possible networks each
and 16777216 end systems (minus special bit values reserved for
network/broadcast addresses); Class B (MSB '10'), with 16384 possible
networks each with 65536 end systems (less reserved values); and
Class C (MSB '110'), and 2097152 possible networks each and 254 end
systems (256 bit combinations minus the reserved all-zeros and all-
ones patterns). The set of addresses with MSB '111' was reserved for
future use; parts of this were eventually defined (MSB '1110') for
use with IPv4 multicast and parts are still reserved as of the
writing of this document.
In the late 1980s, the expansion and commercialization of the former
research network resulted in the connection of many new organizations
to the rapidly growing Internet, and each new organization required
an address assignment according to the Class A/B/C addressing plan.
As demand for new network numbers (particularly in the Class B space)
took what appeared to be an exponential growth rate, some members of
the operations and engineering community started to have concerns
over the long-term scaling properties of the class A/B/C system and
began thinking about how to modify network number assignment policy
and routing protocols to accommodate the growth. In November, 1991,
the Internet Engineering Task Force (IETF) created the ROAD (Routing
and Addressing) group to examine the situation. This group met in
January 1992 and identified three major problems:
1. Exhaustion of the Class B network address space. One fundamental
cause of this problem is the lack of a network class of a size
that is appropriate for mid-sized organization. Class C, with a
maximum of 254 host addresses, is too small, whereas Class B,
which allows up to 65534 host addresses, is too large for most
organizations but was the best fit available for use with
2. Growth of routing tables in Internet routers beyond the ability
of current software, hardware, and people to effectively manage.
3. Eventual exhaustion of the 32-bit IPv4 address space.
It was clear that then-current rates of Internet growth would
cause the first two problems to become critical sometime between
1993 and 1995. Work already in progress on topological
assignment of addressing for Connectionless Network Service
(CLNS), which was presented to the community at the Boulder IETF
in December of 1990, led to thoughts on how to re-structure the
32-bit IPv4 address space to increase its lifespan. Work in the
ROAD group followed and eventually resulted in the publication of
[RFC1338], and later, [RFC1519].
The design and deployment of CIDR was intended to solve these
problems by providing a mechanism to slow the growth of global
routing tables and to reduce the rate of consumption of IPv4
address space. It did not and does not attempt to solve the
third problem, which is of a more long-term nature; instead, it
endeavors to ease enough of the short- to mid-term difficulties
to allow the Internet to continue to function efficiently while
progress is made on a longer-term solution.
More historical background on this effort and on the ROAD group
may be found in [RFC1380] and at [LWRD].
3. Classless Addressing as a Solution
The solution that the community created was to deprecate the Class
A/B/C network address assignment system in favor of using
"classless", hierarchical blocks of IP addresses (referred to as
prefixes). The assignment of prefixes is intended to roughly follow
the underlying Internet topology so that aggregation can be used to
facilitate scaling of the global routing system. One implication of
this strategy is that prefix assignment and aggregation is generally
done according to provider-subscriber relationships, since that is
how the Internet topology is determined.
When originally proposed in [RFC1338] and [RFC1519], this addressing
plan was intended to be a relatively short-term response, lasting
approximately three to five years, during which a more permanent
addressing and routing architecture would be designed and
implemented. As can be inferred from the dates on the original
documents, CIDR has far outlasted its anticipated lifespan and has
become the mid-term solution to the problems described above.
Note that in the following text we describe the current policies and
procedures that have been put in place to implement the allocation
architecture discussed here. This description is not intended to be
interpreted as direction to IANA.
Coupled with address management strategies implemented by the
Regional Internet Registries (see [NRO] for details), the deployment
of CIDR-style addressing has also reduced the rate at which IPv4
address space has been consumed, thus providing short- to medium-term
relief to problem #3, described above.
Note that, as defined, this plan neither requires nor assumes the
re-assignment of those parts of the legacy "Class C" space that are
not amenable to aggregation (sometimes called "the swamp"). Doing so
would somewhat reduce routing table sizes (current estimate is that
"the swamp" contains approximately 15,000 entries), though at a
significant renumbering cost. Similarly, there is no hard
requirement that any end site renumber when changing transit service
provider, but end sites are encouraged do so to eliminate the need
for explicit advertisement of their prefixes into the global routing
3.1. Basic Concept and Prefix Notation
In the simplest sense, the change from Class A/B/C network numbers to
classless prefixes is to make explicit which bits in a 32-bit IPv4
address are interpreted as the network number (or prefix) associated
with a site and which are the used to number individual end systems
within the site. In CIDR notation, a prefix is shown as a 4-octet
quantity, just like a traditional IPv4 address or network number,
followed by the "/" (slash) character, followed by a decimal value
between 0 and 32 that describes the number of significant bits.
For example, the legacy "Class B" network 172.16.0.0, with an implied
network mask of 255.255.0.0, is defined as the prefix 172.16.0.0/16,
the "/16" indicating that the mask to extract the network portion of
the prefix is a 32-bit value where the most significant 16 bits are
ones and the least significant 16 bits are zeros. Similarly, the
legacy "Class C" network number 192.168.99.0 is defined as the prefix
192.168.99.0/24; the most significant 24 bits are ones and the least
significant 8 bits are zeros.
Using classless prefixes with explicit prefix lengths allows much
more flexible matching of address space blocks according to actual
need. Where formerly only three network sizes were available,
prefixes may be defined to describe any power of two-sized block of
between one and 2^32 end system addresses. In practice, the
unallocated pool of addresses is administered by the Internet
Assigned Numbers Authority ([IANA]). The IANA makes allocations from
this pool to Regional Internet Registries, as required. These
allocations are made in contiguous bit-aligned blocks of 2^24
addresses (a.k.a. /8 prefixes). The Regional Internet Registries
(RIRs), in turn, allocate or assign smaller address blocks to Local
Internet Registries (LIRs) or Internet Service Providers (ISPs).
These entities may make direct use of the assignment (as would
commonly be the case for an ISP) or may make further sub-allocations
of addresses to their customers. These RIR address assignments vary
according to the needs of each ISP or LIR. For example, a large ISP
might be allocated an address block of 2^17 addresses (a /15 prefix),
whereas a smaller ISP may be allocated an address block of 2^11
addresses (a /21 prefix).
Note that the terms "allocate" and "assign" have specific meaning in
the Internet address registry system; "allocate" refers to the
delegation of a block of address space to an organization that is
expected to perform further sub-delegations, and "assign" is used for
sites that directly use (i.e., number individual hosts) the block of
The following table provides a convenient shortcut to all the CIDR
prefix sizes, showing the number of addresses possible in each prefix
and the number of prefixes of that size that may be numbered in the
32-bit IPv4 address space:
notation addrs/block # blocks
-------- ----------- ----------
n.n.n.n/32 1 4294967296 "host route"
n.n.n.x/31 2 2147483648 "p2p link"
n.n.n.x/30 4 1073741824
n.n.n.x/29 8 536870912
n.n.n.x/28 16 268435456
n.n.n.x/27 32 134217728
n.n.n.x/26 64 67108864
n.n.n.x/25 128 33554432
n.n.n.0/24 256 16777216 legacy "Class C"
n.n.x.0/23 512 8388608
n.n.x.0/22 1024 4194304
n.n.x.0/21 2048 2097152
n.n.x.0/20 4096 1048576
n.n.x.0/19 8192 524288
n.n.x.0/18 16384 262144
n.n.x.0/17 32768 131072
n.n.0.0/16 65536 65536 legacy "Class B"
n.x.0.0/15 131072 32768
n.x.0.0/14 262144 16384
n.x.0.0/13 524288 8192
n.x.0.0/12 1048576 4096
n.x.0.0/11 2097152 2048
n.x.0.0/10 4194304 1024
n.x.0.0/9 8388608 512
n.0.0.0/8 16777216 256 legacy "Class A"
x.0.0.0/7 33554432 128
x.0.0.0/6 67108864 64
x.0.0.0/5 134217728 32
x.0.0.0/4 268435456 16
x.0.0.0/3 536870912 8
x.0.0.0/2 1073741824 4
x.0.0.0/1 2147483648 2
0.0.0.0/0 4294967296 1 "default route"
n is an 8-bit decimal octet value. Point-to-point links are
discussed in more detail in [RFC3021].
x is a 1- to 7-bit value, based on the prefix length, shifted into
the most significant bits of the octet and converted into decimal
form; the least significant bits of the octet are zero.
In practice, prefixes of length shorter than 8 have not been
allocated or assigned to date, although routes to such short prefixes
may exist in routing tables if or when aggressive aggregation is
performed. As of the writing of this document, no such routes are
seen in the global routing system, but operator error and other
events have caused some of them (i.e., 126.96.36.199/1 and 192.0.0.0/2)
to be observed in some networks at some times in the past.
4. Address Assignment and Routing Aggregation
Classless addressing and routing was initially developed primarily to
improve the scaling properties of routing on the global Internet.
Because the scaling of routing is very tightly coupled to the way
that addresses are used, deployment of CIDR had implications for the
way in which addresses were assigned.
4.1. Aggregation Efficiency and Limitations
The only commonly understood method for reducing routing state on a
packet-switched network is through aggregation of information. For
CIDR to succeed in reducing the size and growth rate of the global
routing system, the IPv4 address assignment process needed to be
changed to make possible the aggregation of routing information along
topological lines. Since, in general, the topology of the network is
determined by the service providers who have built it, topologically
significant address assignments are necessarily service-provider
Aggregation is simple for an end site that is connected to one
service provider: it uses address space assigned by its service
provider, and that address space is a small piece of a larger block
allocated to the service provider. No explicit route is needed for
the end site; the service provider advertises a single aggregate
route for the larger block. This advertisement provides reachability
and routeability for all the customers numbered in the block.
There are two, more complex, situations that reduce the effectiveness
o An organization that is multi-homed. Because a multi-homed
organization must be advertised into the system by each of its
service providers, it is often not feasible to aggregate its
routing information into the address space of any one of those
providers. Note that the organization still may receive its
address assignment out of a service provider's address space
(which has other advantages), but that a route to the
organization's prefix is, in the most general case, explicitly
advertised by all of its service providers. For this reason, the
global routing cost for a multi-homed organization is generally
the same as it was prior to the adoption of CIDR. A more detailed
consideration of multi-homing practices can be found in [RFC4116].
o An organization that changes service provider but does not
renumber. This has the effect of "punching a hole" in one of the
original service provider's aggregated route advertisements. CIDR
handles this situation by requiring that the newer service
provider to advertise a specific advertisement for the re-homed
organization; this advertisement is preferred over provider
aggregates because it is a longer match. To maintain efficiency
of aggregation, it is recommended that an organization that
changes service providers plan eventually to migrate its network
into a an prefix assigned from its new provider's address space.
To this end, it is recommended that mechanisms to facilitate such
migration, such as dynamic host address assignment that uses
[RFC2131]), be deployed wherever possible, and that additional
protocol work be done to develop improved technology for
Note that some aggregation efficiency gain can still be had for
multi-homed sites (and, in general, for any site composed of
multiple, logical IPv4 networks); by allocating a contiguous power-
of-two block address space to the site (as opposed to multiple,
independent prefixes), the site's routing information may be
aggregated into a single prefix. Also, since the routing cost
associated with assigning a multi-homed site out of a service
provider's address space is no greater than the old method of
sequential number assignment by a central authority, it makes sense
to assign all end-site address space out of blocks allocated to
It is also worthwhile to mention that since aggregation may occur at
multiple levels in the system, it may still be possible to aggregate
these anomalous routes at higher levels of whatever hierarchy may be
present. For example, if a site is multi-homed to two relatively
small providers that both obtain connectivity and address space from
the same large provider, then aggregation by the large provider of
routes from the smaller networks will include all routes to the
multi-homed site. The feasibility of this sort of second-level
aggregation depends on whether topological hierarchy exists among a
site, its directly-connected providers, and other providers to which
they are connected; it may be practical in some regions of the global
Internet but not in others.
Note: In the discussion and examples that follow, prefix notation is
used to represent routing destinations. This is used for
illustration only and does not require that routing protocols use
this representation in their updates.
4.2. Distributed Assignment of Address Space
In the early days of the Internet, IPv4 address space assignment was
performed by the central Network Information Center (NIC). Class
A/B/C network numbers were assigned in essentially arbitrary order,
roughly according to the size of the organizations that requested
them. All assignments were recorded centrally, and no attempt was
made to assign network numbers in a manner that would allow routing
When CIDR was originally deployed, the central assignment authority
continued to exist but changed its procedures to assign large blocks
of "Class C" network numbers to each service provider. Each service
provider, in turn, assigned bitmask-oriented subsets of the
provider's address space to each customer. This worked reasonably
well, as long as the number of service providers was relatively small
and relatively constant, but it did not scale well, as the number of
service providers grew at a rapid rate.
As the Internet started to expand rapidly in the 1990s, it became
clear that a single, centralized address assignment authority was
problematic. This function began being de-centralized when address
space assignment for European Internet sites was delegated in bit-
aligned blocks of 16777216 addresses (what CIDR would later define as
a /8) to the RIPE NCC ([RIPE]), effectively making it the first of
the RIRs. Since then, address assignment has been formally
distributed as a hierarchical function with IANA, the RIRs, and the
service providers. Removing the bottleneck of a single organization
having responsibility for the global Internet address space greatly
improved the efficiency and response time for new assignments.
Hierarchical delegation of addresses in this manner implies that
sites with addresses assigned out of a given service provider are,
for routing purposes, part of that service provider and will be
routed via its infrastructure. This implies that routing information
about multi-homed organizations (i.e., organizations connected to
more than one network service provider) will still need to be known
by higher levels in the hierarchy.
A historical perspective on these issues is described in [RFC1518].
Additional discussion may also be found in [RFC3221].
5. Routing Implementation Considerations
With the change from classful network numbers to classless prefixes,
it is not possible to infer the network mask from the initial bit
pattern of an IPv4 address. This has implications for how routing
information is stored and propagated. Network masks or prefix
lengths must be explicitly carried in routing protocols. Interior
routing protocols, such as OSPF [RFC2328], Intermediate System to
Intermediate System (IS-IS) [RFC1195], RIPv2 [RFC2453], and Cisco
Enhanced Interior Gateway Routing Protocol (EIGRP), and the BGP4
exterior routing protocol [RFC4271], all support this functionality,
having been developed or modified as part of the deployment of
classless inter-domain routing during the 1990s.
Older interior routing protocols, such as RIP [RFC1058], HELLO, and
Cisco Interior Gateway Routing Protocol (IGRP), and older exterior
routing protocols, such as Exterior Gateway Protocol (EGP) [RFC904],
do not support explicit carriage of prefix length/mask and thus
cannot be effectively used on the Internet other than in very limited
stub configurations. Although their use may be appropriate in simple
legacy end-site configurations, they are considered obsolete and
should NOT be used in transit networks connected to the global
Similarly, routing and forwarding tables in layer-3 network equipment
must be organized to store both prefix and prefix length or mask.
Equipment that organizes its routing/forwarding information according
to legacy Class A/B/C network/subnet conventions cannot be expected
to work correctly on networks connected to the global Internet; use
of such equipment is not recommended. Fortunately, very little such
equipment is in use today.
5.1. Rules for Route Advertisement
1. Forwarding in the Internet is done on a longest-match basis.
This implies that destinations that are multi-homed relative to a
routing domain must always be explicitly announced into that
routing domain (i.e., they cannot be summarized). If a network
is multi-homed, all of its paths into a routing domain that is
"higher" in the hierarchy of networks must be known to the
2. A router that generates an aggregate route for multiple, more-
specific routes must discard packets that match the aggregate
route, but not any of the more-specific routes. In other words,
the "next hop" for the aggregate route should be the null
destination. This is necessary to prevent forwarding loops when
some addresses covered by the aggregate are not reachable.
Note that during failures, partial routing of traffic to a site that
takes its address space from one service provider but that is
actually reachable only through another (i.e., the case of a site
that has changed service providers) may occur because such traffic
will be forwarded along the path advertised by the aggregated route.
Rule #2 will prevent packet misdelivery by causing such traffic to be
discarded by the advertiser of the aggregated route, but the output
of "traceroute" and other similar tools will suggest that a problem
exists within that network rather than in the network that is no
longer advertising the more-specific prefix. This may be confusing
to those trying to diagnose connectivity problems; see the example in
Section 6.2 for details. A solution to this perceived "problem" is
beyond the scope of this document; it lies with better education of
the user/operator community, not in routing technology.
An implementation following these rules should also be generalized,
so that an arbitrary network number and mask are accepted for all
routing destinations. The only outstanding constraint is that the
mask must be left contiguous. Note that the degenerate route to
prefix 0.0.0.0/0 is used as a default route and MUST be accepted by
all implementations. Further, to protect against accidental
advertisements of this route via the inter-domain protocol, this
route should only be advertised to another routing domain when a
router is explicitly configured to do so, never as a non-configured,
5.2. How the Rules Work
Rule #1 guarantees that the forwarding algorithm used is consistent
across routing protocols and implementations. Multi-homed networks
are always explicitly advertised by every service provider through
which they are routed, even if they are a specific subset of one
service provider's aggregate (if they are not, they clearly must be
explicitly advertised). It may seem as if the "primary" service
provider could advertise the multi-homed site implicitly as part of
its aggregate, but longest-match forwarding causes this not to work.
More details are provided in [RFC4116].
Rule #2 guarantees that no routing loops form due to aggregation.
Consider a site that has been assigned 192.168.64/19 by its "parent"
provider, which has 192.168.0.0/16. The "parent" network will
advertise 192.168.0.0/16 to the "child" network. If the "child"
network were to lose internal connectivity to 192.168.65.0/24 (which
is part of its aggregate), traffic from the "parent" to the to the
"child" destined for 192.168.65.1 will follow the "child's"
advertised route. When that traffic gets to the "child", however,
the child *must not* follow the route 192.168.0.0/16 back up to the
"parent", since that would result in a forwarding loop. Rule #2 says
that the "child" may not follow a less-specific route for a
destination that matches one of its own aggregated routes (typically,
this is implemented by installing a "discard" or "null" route for all
aggregated prefixes that one network advertises to another). Note
that handling of the "default" route (0.0.0.0/0) is a special case of
this rule; a network must not follow the default to destinations that
are part of one of its aggregated advertisements.
5.3. A Note on Prefix Filter Formats
Systems that process route announcements must be able to verify that
information that they receive is acceptable according to policy
rules. Implementations that filter route advertisements must allow
masks or prefix lengths in filter elements. Thus, filter elements
that formerly were specified as
now look something like this:
This is merely making explicit the network mask that was implied by
the Class A/B/C classification of network numbers. It is also useful
to enhance filtering capability to allow the match of a prefix and
all more-specific prefixes with the same bit pattern; fortunately,
this functionality has been implemented by most vendors of equipment
used on the Internet.
5.4. Responsibility for and Configuration of Aggregation
Under normal circumstances, a routing domain (or "Autonomous System")
that has been allocated or assigned a set of prefixes has sole
responsibility for aggregation of those prefixes. In the usual case,
the AS will install configuration in one or more of its routers to
generate aggregate routes based on more-specific routes known to its
internal routing system. These aggregate routes are advertised into
the global routing system by the border routers for the routing
domain. The more-specific internal routes that overlap with the
aggregate routes should not be advertised globally. In some cases,
an AS may wish to delegate aggregation responsibility to another AS
(for example, a customer may wish for its service provider to
generate aggregated routing information on its behalf); in such
cases, aggregation is performed by a router in the second AS
according to the routes that it receives from the first, combined
with configured policy information describing how those routes should
Note that one provider may choose to perform aggregation on the
routes it receives from another without explicit agreement; this is
termed "proxy aggregation". This can be a useful tool for reducing
the amount of routing state that an AS must carry and propagate to
its customers and neighbors. However, proxy aggregation can also
create unintended consequences in traffic engineering. Consider what
happens if both AS 2 and 3 receive routes from AS 1 but AS 2 performs
proxy aggregation while AS 3 does not. Other ASes that receive
transit routing information from both AS 2 and AS 3 will see an
inconsistent view of the routing information originated by AS 1.
This may cause an unexpected shift of traffic toward AS 1 through AS
3 for AS 3's customers and any others receiving transit routes from
AS 3. Because proxy aggregation can cause unanticipated consequences
for parts of the Internet that have no relationship with either the
source of the aggregated routes or the party providing aggregation,
it should be used with extreme caution.
Configuration of the routes to be combined into aggregates is an
implementation of routing policy and requires some manually
maintained information. As an addition to the information that must
be maintained for a set of routeable prefixes, aggregation
configuration is typically just a line or two defining the range of
the block of IPv4 addresses to be aggregated. A site performing its
own aggregation is doing so for address blocks that it has been
assigned; a site performing aggregation on behalf of another knows
this information because of an agreement to delegate aggregation.
Assuming that the best common practice for network administrators is
to exchange lists of prefixes to accept from each other,
configuration of aggregation information does not introduce
significant additional administrative overhead.
The generation of an aggregate route is usually specified either
statically or in response to learning an active dynamic route for a
prefix contained within the aggregate route. If such dynamic
aggregate route advertisement is done, care should be taken that
routes are not excessively added or withdrawn (known as "route
flapping"). In general, a dynamic aggregate route advertisement is
added when at least one component of the aggregate becomes reachable
and it is withdrawn only when all components become unreachable.
Properly configured, aggregated routes are more stable than non-
aggregated routes and thus improve global routing stability.
Implementation note: Aggregation of the "Class D" (multicast) address
space is beyond the scope of this document.
5.5. Route Propagation and Routing Protocol Considerations
Prior to the original deployment of CIDR, common practice was to
propagate routes learned via exterior routing protocols (i.e., EGP or
BGP) through a site's interior routing protocol (typically, OSPF,
IS-IS, or RIP). This was done to ensure that consistent and correct
exit points were chosen for traffic to be sent to a destination
learned through those protocols. Four evolutionary effects -- the
advent of CIDR, explosive growth of global routing state, widespread
adoption of BGP4, and a requirement to propagate full path
information -- have combined to deprecate that practice. To ensure
proper path propagation and prevent inter-AS routing inconsistency
(BGP4's loop detection/prevention mechanism requires full path
propagation), transit networks must use internal BGP (iBGP) for
carrying routes learned from other providers both within and through
6. Example of New Address Assignments and Routing
6.1. Address Delegation
Consider the block of 524288 (2^19) addresses, beginning with
10.24.0.0 and ending with 10.31.255.255, allocated to a single
network provider, "PA". This is equivalent in size to a block of
2048 legacy "Class C" network numbers (or /24s). A classless route
to this block would be described as 10.24.0.0 with a mask of
255.248.0.0 and the prefix 10.24.0.0/13.
Assume that this service provider connects six sites in the following
order (significant because it demonstrates how temporary "holes" may
form in the service provider's address space):
o "C1", requiring fewer than 2048 addresses (/21 or 8 x /24)
o "C2", requiring fewer than 4096 addresses (/20 or 16 x /24)
o "C3", requiring fewer than 1024 addresses (/22 or 4 x /24)
o "C4", requiring fewer than 1024 addresses (/22 or 4 x /24)
o "C5", requiring fewer than 512 addresses (/23 or 2 x /24)
o "C6", requiring fewer than 512 addresses (/23 or 2 x /24)
In all cases, the number of IPv4 addresses "required" by each site is
assumed to allow for significant growth. The service provider
delegates its address space as follows:
o C1. assign 10.24.0 through 10.24.7. This block of networks is
described by the route 10.24.0.0/21 (mask 255.255.248.0).
o C2. Assign 10.24.16 through 10.24.31. This block is described by
the route 10.24.16.0/20 (mask 255.255.240.0).
o C3. Assign 10.24.8 through 10.24.11. This block is described by
the route 10.24.8.0/22 (mask 255.255.252.0).
o C4. Assign 10.24.12 through 10.24.15. This block is described by
the route 10.24.12.0/22 (mask 255.255.252.0).
o C5. Assign 10.24.32 and 10.24.33. This block is described by the
route 10.24.32.0/23 (mask 255.255.254.0).
o C6. Assign 10.24.34 and 10.24.35. This block is described by the
route 10.24.34.0/23 (mask 255.255.254.0).
These six sites should be represented as six prefixes of varying size
within the provider's IGP. If, for some reason, the provider uses an
obsolete IGP that doesn't support classless routing or variable-
length subnets, then explicit routes for all /24s will have to be
To make this example more realistic, assume that C4 and C5 are multi-
homed through some other service provider, "PB". Further assume the
existence of a site, "C7", that was originally connected to "RB" but
that has moved to "PA". For this reason, it has a block of network
numbers that are assigned out PB's block of (the next) 2048 x /24.
o C7. Assign 10.32.0 through 10.32.15. This block is described by
the route 10.32.0.0/20 (mask 255.255.240.0).
For the multi-homed sites, assume that C4 is advertised as primary
via "RA" and secondary via "RB"; and that C5 is primary via "RB" and
secondary via "RA". In addition, assume that "RA" and "RB" are both
connected to the same transit service provider, "BB".
Graphically, this topology looks something like this:
10.24.0.0 -- 10.24.7.0__ __10.32.0.0 - 10.32.15.0
C1: 10.24.0.0/21 \ / C7: 10.32.0.0/20
10.24.16.0 - 10.24.31.0_ | | | |
C2: 10.24.16.0/20 \ | | _10.24.12.0 - 10.24.15.0__ | |
\| | / C4: 10.24.12.0/20 \ | |
| |/ \| |
10.24.8.0 - 10.24.11.0___/| PA |\ | PB |
C3: 10.24.8.0/22 | | \__10.24.32.0 - 10.24.33.0___| |
| | C5: 10.24.32.0/23 | |
| | | |
10.24.34.0 - 10.24.35.0__/| | | |
C6: 10.24.34.0/23 | | | |
routing advertisements: || ||
10.24.12.0/22 (C4) || 10.24.12.0/22 (C4) ||
10.32.0.0/20 (C7) || 10.24.32.0/23 (C5) ||
10.24.0.0/13 (PA) || 10.32.0.0/13 (PB) ||
+---------- BACKBONE NETWORK BB ----------+
6.2. Routing Advertisements
To follow rule #1, PA will need to advertise the block of addresses
that it was given and C7. Since C4 is multi-homed and primary
through PA, it must also be advertised. C5 is multi-homed and
primary through PB. In principle (and in the example above), it need
not be advertised, since longest match by PB will automatically
select PB as primary and the advertisement of PA's aggregate will be
used as a secondary. In actual practice, C5 will normally be
advertised via both providers.
Advertisements from "PA" to "BB" will be
10.24.12.0/22 primary (advertises C4)
10.32.0.0/20 primary (advertises C7)
10.24.0.0/13 primary (advertises remainder of PA)
For PB, the advertisements must also include C4 and C5, as well as
its block of addresses.
Advertisements from "PB" to "BB" will be
10.24.12.0/22 secondary (advertises C4)
10.24.32.0/23 primary (advertises C5)
10.32.0.0/13 primary (advertises remainder of RB)
To illustrate the problem diagnosis issue mentioned in Section 5.1,
consider what happens if PA loses connectivity to C7 (the site that
is assigned out of PB's space). In a stateful protocol, PA will
announce to BB that 10.32.0.0/20 has become unreachable. Now, when
BB flushes this information out of its routing table, any future
traffic sent through it for this destination will be forwarded to PB
(where it will be dropped according to Rule #2) by virtue of PB's
less-specific match, 10.32.0.0/13. Although this does not cause an
operational problem (C7 is unreachable in any case), it does create
some extra traffic across "BB" (and may also prove confusing to
someone trying to debug the outage with "traceroute"). A mechanism
to cache such unreachable state might be nice, but it is beyond the
scope of this document.
7. Domain Name Service Considerations
One aspect of Internet services that was notably affected by the move
to CIDR was the mechanism used for address-to-name translation: the
IN-ADDR.ARPA zone of the domain system. Because this zone is
delegated on octet boundaries only, the move to an address assignment
plan that uses bitmask-oriented addressing caused some increase in
work for those who maintain parts of the IN-ADDR.ARPA zone.
A description of techniques to populate the IN-ADDR.ARPA zone when
and used address that blocks that do not align to octet boundaries is
described in [RFC2317].
8. Transition to a Long-Term Solution
CIDR was designed to be a short-term solution to the problems of
routing state and address depletion on the IPv4 Internet. It does
not change the fundamental Internet routing or addressing
architectures. It is not expected to affect any plans for transition
to a more long-term solution except, perhaps, by delaying the urgency
of developing such a solution.
9. Analysis of CIDR's Effect on Global Routing State
When CIDR was first proposed in the early 1990s, the original authors
made some observations about the growth rate of global routing state
and offered projections on how CIDR deployment would, hopefully,
reduce what appeared to be exponential growth to a more sustainable
rate. Since that deployment, an ongoing effort, called "The CIDR
Report" [CRPT], has attempted to quantify and track that growth rate.
What follows is a brief summary of the CIDR report as of March 2005,
with an attempt to explain the various patterns and changes of growth
rate that have occurred since measurements of the size of global
routing state began in 1988.
When the graph of "Active BGP Table Entries" [CBGP] is examined,
there appear to be several different growth trends with distinct
inflection points reflecting changes in policy and practice. The
trends and events that are believed to have caused them were as
1. Exponential growth at the far left of the graph. This represents
the period of early expansion and commercialization of the former
research network, from the late 1980s through approximately 1994.
The major driver for this growth was a lack of aggregation
capability for transit providers, and the widespread use of
legacy Class C allocations for end sites. Each time a new site
was connected to the global Internet, one or more new routing
entries were generated.
2. Acceleration of the exponential trend in late 1993 and early 1994
as CIDR "supernet" blocks were first assigned by the NIC and
routed as separate legacy class-C networks by service provider.
3. A sharp drop in 1994 as BGP4 deployment by providers allowed
aggregation of the "supernet" blocks. Note that the periods of
largest declines in the number of routing table entries typically
correspond to the weeks following each meeting of the IETF CIDR
Deployment Working Group.
4. Roughly linear growth from mid-1994 to early 1999 as CIDR-based
address assignments were made and aggregated routes added
throughout the network.
5. A new period of exponential growth again from early 1999 until
2001 as the "high-tech bubble" fueled both rapid expansion of the
Internet, as well as a large increase in more-specific route
advertisements for multi-homing and traffic engineering.
6. Flattening of growth through 2001 caused by a combination of the
"dot-com bust", which caused many organizations to cease
operations, and the "CIDR police" [CPOL] work aimed at improving
7. Roughly linear growth through 2002 and 2003. This most likely
represents a resumption of the "normal" growth rate observed
before the "bubble", as well as an end to the "CIDR Police"
8. A more recent trend of exponential growth beginning in 2004. The
best explanation would seem to be an improvement of the global
economy driving increased expansion of the Internet and the
continued absence of the "CIDR Police" effort, which previously
served as an educational tool for new providers to improve
aggregation efficiency. There have also been some cases where
service providers have deliberately de-aggregated prefixes in an
attempt to mitigate security problems caused by conflicting route
advertisements (see Section 12). Although this behavior may
solve the short-term problems seen by such providers, it is
fundamentally non-scalable and quite detrimental to the community
as a whole. In addition, there appear to be many providers
advertising both their allocated prefixes and all the /24
components thereof, probably due to a lack of consistent current
information about recommended routing configuration.
10. Conclusions and Recommendations
In 1992, when CIDR was first developed, there were serious problems
facing the continued growth of the Internet. Growth in routing state
complexity and the rapid increase in consumption of address space
made it appear that one or both problems would preclude continued
growth of the Internet within a few short years.
Deployment of CIDR, in combination with BGP4's support for carrying
classless prefix routes, alleviated the short-term crisis. It was
only through a concerted effort by both the equipment manufacturers
and the provider community that this was achieved. The threat (and,
perhaps in some cases, actual implementation of) charging networks
for advertising prefixes may have offered an additional incentive to
share the address space, and thus the associated costs of advertising
routes to service providers.
The IPv4 routing system architecture carries topology information
based on aggregate address advertisements and a collection of more-
specific advertisements that are associated with traffic engineering,
multi-homing, and local configuration. As of March 2005, the base
aggregate address load in the routing system has some 75,000 entries.
Approximately 85,000 additional entries are more specific entries of
this base "root" collection. There is reason to believe that many of
these additional entries exist to solve problems of regional or even
local scope and should not need to be globally propagated.
An obvious question to ask is whether CIDR can continue to be a
viable approach to keeping global routing state growth and address
space depletion at sustainable rates. Recent measurements indicate
that exponential growth has resumed, but further analysis suggests
that this trend can be mitigated by a more active effort to educate
service providers as to efficient aggregation strategies and proper
equipment configuration. Looking farther forward, there is a clear
need for better multi-homing technology that does not require global
routing state for each site and for methods of performing traffic
load balancing that do not require adding even more state. Without
such developments and in the absence of major architectural change,
aggregation is the only tool available for making routing scale in
the global Internet.
11. Status Updates to CIDR Documents
This memo renders obsolete and requests re-classification as Historic
the following RFCs describing CIDR usage and deployment:
o RFC 1467: Status of CIDR Deployment in the Internet
This Informational RFC described the status of CIDR deployment in
1993. As of 2005, CIDR has been thoroughly deployed, so this
status note only provides a historical data point.
o RFC 1481: IAB Recommendation for an Intermediate Strategy to
Address the Issue of Scaling
This very short Informational RFC described the IAB's endorsement
of the use of CIDR to address scaling issues. Because the goal of
RFC 1481 has been achieved, it is now only of historical value.
o RFC 1482: Aggregation Support in the NSFNET Policy-Based Routing
This Informational RFC describes plans for support of route
aggregation, as specified by CIDR, on the NSFNET. Because the
NSFNET has long since ceased to exist and CIDR has been
ubiquitously deployed, RFC 1482 now only has historical relevance.
o RFC 1517: Applicability Statement for the Implementation of
Classless Inter-Domain Routing (CIDR)
This Standards Track RFC described where CIDR was expected to be
required and where it was expected to be (strongly) recommended.
With the full deployment of CIDR on the Internet, situations where
CIDR is not required are of only historical interest.
o RFC 1518: An Architecture for IP Address Allocation with CIDR
This Standards Track RFC discussed routing and address aggregation
considerations at some length. Some of these issues are
summarized in this document in section Section 3.1. Because
address assignment policies and procedures now reside mainly with
the RIRs, it is not appropriate to try to document those practices
in a Standards Track RFC. In addition, [RFC3221] also describes
many of the same issues from point of view of the routing system.
o RFC 1520: Exchanging Routing Information Across Provider
Boundaries in the CIDR Environment
This Informational RFC described transition scenarios where CIDR
was not fully supported for exchanging route information between
providers. With the full deployment of CIDR on the Internet, such
scenarios are no longer operationally relevant.
o RFC 1817: CIDR and Classful Routing
This Informational RFC described the implications of CIDR
deployment in 1995; it notes that formerly-classful addresses were
to be allocated using CIDR mechanisms and describes the use of a
default route for non-CIDR-aware sites. With the full deployment
of CIDR on the Internet, such scenarios are no longer
o RFC 1878: Variable Length Subnet Table For IPv4
This Informational RFC provided a table of pre-calculated subnet
masks and address counts for each subnet size. With the
incorporation of a similar table into this document (see Section
3.1), it is no longer necessary to document it in a separate RFC.
o RFC 2036: Observations on the use of Components of the Class A
Address Space within the Internet
This Informational RFC described several operational issues
associated with the allocation of classless prefixes from
previously-classful address space. With the full deployment of
CIDR on the Internet and more than half a dozen years of
experience making classless prefix allocations out of historical
"Class A" address space, this RFC now has only historical value.
12. Security Considerations
The introduction of routing protocols that support classless prefixes
and a move to a forwarding model that mandates that more-specific
(longest-match) routes be preferred when they overlap with routes to
less-specific prefixes introduces at least two security concerns:
1. Traffic can be hijacked by advertising a prefix for a given
destination that is more specific than the aggregate that is
normally advertised for that destination. For example, assume
that a popular end system with the address 192.168.17.100 is
connected to a service provider that advertises 192.168.16.0/20.
A malicious network operator interested in intercepting traffic
for this site might advertise, or at least attempt to advertise,
192.168.17.0/24 into the global routing system. Because this
prefix is more specific than the "normal" prefix, traffic will be
diverted away from the legitimate end system and to the network
owned by the malicious operator. Prior to the advent of CIDR, it
was possible to induce traffic from some parts of the network to
follow a false advertisement that exactly matched a particular
network number; CIDR makes this problem somewhat worse, since
longest-match routing generally causes all traffic to prefer
more-specific routes over less-specific routes. The remedy for
the CIDR-based attack, though, is the same as for a pre-CIDR-
based attack: establishment of trust relationships between
providers, coupled with and strong route policy filters at
provider borders. Unfortunately, the implementation of such
filters is difficult in the highly de-centralized Internet. As a
workaround, many providers do implement generic filters that set
upper bounds, derived from RIR guidelines for the sizes of blocks
that they allocate, on the lengths of prefixes that are accepted
from other providers. Note that "spammers" have been observed
using this sort of attack to hijack address space temporarily in
order to hide the origin of the traffic ("spam" email messages)
that they generate.
2. Denial-of-service attacks can be launched against many parts of
the Internet infrastructure by advertising a large number of
routes into the system. Such an attack is intended to cause
router failures by overflowing routing and forwarding tables. A
good example of a non-malicious incident that caused this sort of
failure was the infamous "AS 7007" event , where a router
mis-configuration by an operator caused a huge number of invalid
routes to be propagated through the global routing system.
Again, this sort of attack is not really new with CIDR; using
legacy Class A/B/C routes, it was possible to advertise a maximum
of 16843008 unique network numbers into the global routing
system, a number that is sufficient to cause problems for even
the most modern routing equipment made in 2005. What is
different is that the moderate complexity of correctly
configuring routers in the presence of CIDR tends to make
accidental "attacks" of this sort more likely. Measures to
prevent this sort of attack are much the same as those described
above for the hijacking, with the addition that best common
practice is also to configure a reasonable maximum number of
prefixes that a border router will accept from its neighbors.
Note that this is not intended to be an exhaustive analysis of the
sorts of attacks that CIDR makes easier; a more comprehensive
analysis of security vulnerabilities in the global routing system is
beyond the scope of this document.
The authors wish to express appreciation to the other original
authors of RFC 1519 (Kannan Varadhan, Jessica Yu); to the ROAD group,
with whom many of the ideas behind CIDR were inspired and developed;
and to the early reviewers of this re-spun version of the document
(Barry Greene, Danny McPherson, Dave Meyer, Eliot Lear, Bill Norton,
Ted Seely, Philip Smith, Pekka Savola), whose comments, corrections,
and suggestions were invaluable. We would especially like to thank
Geoff Huston for contributions well above and beyond the call of
14.1. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
14.2. Informative References
 "NANOG mailing list discussion of the "AS 7007" incident",
[CBGP] "Graph: Active BGP Table Entries, 1988 to Present",
[CPOL] "CIDR Police - Please Pull Over and Show Us Your BGP",
[CRPT] "The CIDR Report", <http://www.cidr-report.org/>.
[IANA] "Internet Assigned Numbers Authority",
[LWRD] "The Long and Winding Road",
[NRO] "Number Resource Organization", <http://www.nro.net>.
[RFC904] Mills, D., "Exterior Gateway Protocol formal
specification", RFC 904, April 1 1984.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058,
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC1338] Fuller, V., Li, T., Yu, J., and K. Varadhan,
"Supernetting: an Address Assignment and Aggregation
Strategy", RFC 1338, June 1992.
[RFC1380] Gross, P. and P. Almquist, "IESG Deliberations on Routing
and Addressing", RFC 1380, November 1992.
[RFC1518] Rekhter, Y. and T. Li, "An Architecture for IP Address
Allocation with CIDR", RFC 1518, September 1993.
[RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC 1519, September 1993.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
[RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson,
"Using 31-Bit Prefixes on IPv4 Point-to-Point Links", RFC
3021, December 2000.
[RFC3221] Huston, G., "Commentary on Inter-Domain Routing in the
Internet", RFC 3221, December 2001.
[RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
Gill, "IPv4 Multihoming Practices and Limitations", RFC
4116, July 2005.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RIPE] "RIPE Network Coordination Centre", <http://www.ripe.net>.
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