|Title||A Two-Tier Address Structure for the Internet: A Solution to the
Problem of Address Space Exhaustion
|Author||Z. Wang, J. Crowcroft
Network Working Group Z. Wang
Request for Comments: 1335 J. Crowcroft
University College London
A Two-Tier Address Structure for the Internet:
A Solution to the Problem of Address Space Exhaustion
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
This RFC presents a solution to problem of address space exhaustion
in the Internet. It proposes a two-tier address structure for the
Internet. This is an "idea" paper and discussion is strongly
Address space exhaustion is one of the most serious and immediate
problems that the Internet faces today [1,2]. The current Internet
address space is 32-bit. Each Internet address is divided into two
parts: a network portion and a host portion. This division
corresponds the three primary Internet address classes: Class A,
Class B and Class C. Table 1 lists the network number statistics as
of April 1992.
Total Allocated Allocated (%)
Class A 126 48 54%
Class B 16383 7006 43%
Class C 2097151 40724 2%
Table 1: Network Number Statistics (April 1992)
If recent trends of exponential growth continue, the network numbers
in Class B will soon run out [1,2]. There are over 2 million Class C
network numbers and only 2% have been allocated. However, a Class C
network number can only accommodate 254 host numbers which is too
small for most networks. With the rapid expansion of the Internet
and drastic increase in personal computers, the time when the 32-bit
address space is exhausted altogether is also not too distant [1-3].
Recently several proposals have been put forward to deal with the
immediate problem [1-4]. The Supernetting and C-sharp schemes
attempt to make the Class C numbers more usable by re-defining the
way in which Class C network numbers are classified and assigned
[3,4]. Both schemes require modifications to the exterior routing
algorithms and global coordination across the Internet may be
required for the deployment. The two schemes do not expand the total
number of addresses available to the Internet and therefore can only
be used as a short-term fix for next two or three years. Schemes
have also been put forwarded in which the 32-bit address field is
replaced with a field of the same size but with different meaning and
the gateways on the boundary re-write the address when the packet
crossed the boundary [1,2,5]. Such schemes, however, requires
substantial changes to the gateways and the exterior routing
In this paper, we present an alternative solution to the problem of
address space exhaustion. The "Dual Network Addressing (DNA)" scheme
proposed here is based on a two-tier address structure and sharing of
addresses. It requires no modifications to the exterior routing
algorithms and any networks can adopt the scheme individually at any
time without affecting other networks.
The DNA scheme attempts to reduce the waste in using the Internet
addresses. A useful analogy to our scheme is the extension system
used in the telephone system. Many large organizations usually have
extensive private telephone networks for internal use and at the mean
time hire a limited number of external lines for communications with
the outside world. In such a telephone system, important offices may
have direct external lines and telephones in the public areas may be
restricted to internal calls only. The majority of the telephones
can usually make both internal calls and external calls. But they
must share a limited number of external lines. When an external call
is being made, a pre-defined digit has to be pressed so that an
external line can be allocated from the poll of external lines.
In the DNA scheme, there are two types of Internet addresses:
Internal addresses and External addresses. An internal address is an
Internet address only used within one network and is unique only
within that network. An interface with an internal address can only
communicate with another interface with an internal address in the
same network. An external address is unique in the entire Internet
and an interface with an external address can communicate directly to
another interface with an external address over the Internet. All
current Internet addresses are external addresses.
In effect, the external addresses form one global Internet and the
internal addresses form many private Internets. Within one network,
the external addresses are only used for inter-network communications
and internal addresses for intra-network communications. An External
Address Sharing Service (EASS) is needed to manage the sharing of
external addresses. An EASS server reserves a number of external
addresses. When a machine that only has an internal address wants to
communicate a machine with an external address in other networks, it
can send a request to an EASS server to obtain a temporary external
address. After the use, the machine can return the external address
to the EASS server.
We believe that, with the DNA scheme, a network can operate with a
limited number of external addresses. The reasons are as follows:
* In most networks, the majority of the traffic is confined to
its local area networks. This is due the nature of
networking applications and the bandwidth constraints on
* The number of machines which act as Internet servers, i.e.,
running programs waiting to be called by machines in other
networks, is often limited and certainly much smaller than
the total number of machines. These machines include mail
servers, domain name servers, ftp archive servers, directory
* There are an increasingly large number of personal machines
entering the Internet. The use of these machines is
primarily limited to their local environment. They may also
be used as "clients" such as ftp and telnet to access other
* For security reasons, many large organizations, such as banks,
government departments, military institution and some
companies, may only allow a very limited number of their
machines to have access to the global Internet. The majority
of their machines are purely for internal use.
In the DNA scheme, all machines in a network are assigned a permanent
internal address and can communicate with any machines within the
same network. The allocation of external addresses depends on the
functions of the machines and as a result it creates three-level
* machines which act as servers or used as central computing
infrastructure are likely to have frequent communications
with other networks therefore they may require external
addresses all the time. These machines are allocated
permanent external addresses.
* machines which are not allowed to communicate with other
networks have no external addresses and can only communicate
with machines within their own network.
* the rest of the machines share a number of external
addresses. The external addresses are allocated by
the EASS server on request. These machines can only
used as clients to call machines in other networks,
i.e., they can not be called by machines in other networks.
A network can choose any network number other than its external
network number as its internal network number. Different networks
can use the same network number as their internal number. We propose
to reserve one Class A network number as the well-known network
number for internal use.
The DNA scheme attempts to tackle the problem from the bottom of the
Internet, i.e., each individual network, while other schemes
described in the first section deal with the problem from the top of
the Internet, i.e., gateways and exterior routing algorithms. These
schemes, however, do not need to be consider as mutually exclusive.
The DNA scheme has several advantages:
* The DNA scheme takes an evolutionary approach towards the
changes. Different networks can individually choose to
adopt the scheme at any time only when necessary.
There is no need for global coordination between different
networks for their deployment. The effects of the deployment
are confined to the network in which the scheme is being
implemented, and are invisible to exterior routing
algorithms and external networks.
* With the DNA scheme, it is possible for a medium size organization
to use a Class C network number with 254 external addresses.
The scheme allows the current Internet to expand to over 2 million
networks and each network to have more than 16 million hosts.
This will allow considerable time for a long-term solution to
be developed and fully tested.
* The DNA scheme requires modifications to the host software.
However, the modifications are needed only in those networks
which adopt the DNA scheme. Since all existing Class A and B
networks usually have sufficient external addresses for all their
machines, they do not need to adopt the DNA scheme, and therefore
need no modifications at all to their software. The networks
which need to use the DNA scheme are those new networks which are
set up after the Class A and B numbers run out and have to
use a Class C number.
* The DNA scheme makes it possible to develop to a new addressing
scheme without expanding the 32-bit address length to 64-bit.
With the two-tier address structure, the current 32-bit space
can accommodate over 4 billion hosts in the global Internet and
100 million hosts in each individual network. When we move to a
classless multi-hierarchic addressing scheme, the use of external
addresses can be more efficient and less wasteful and the
32-bit space can be adequate for the external addresses.
* When a new addressing scheme has been developed, all current
Internet addresses have to be changed. The DNA scheme will make
such a undertaking much easier and smoother, since only the
EASS servers and those have permanent external addresses will
be affected, and communications within the network will not
The major modifications to the host software is in the network
interface code. The DNA scheme requires each machine to have at
least two addresses. But most of the host software currently does
not allow us to bind two addresses to one physical interface. This
problem can be solved by using two network interfaces on each
machine. But this option is too expensive. Note the two interfaces
are actually connected to the same physical network. Therefore, if
we modify the interface code to allow two logical interfaces to be
mapped onto one single physical interface, the machine can then use
both the external address and the internal address with one physical
interface as if it has two physical interfaces. In effect, two
logical IP networks operate over the same physical network.
The DNA scheme also has implications to the DNS service. Many
machines will have two entries in the local name server. The DNS
server must examine the source address of the request and decide
which entry to use. If the source address matches the well-known
internal network number, it passes the internal address of the domain
name. Otherwise, the name server passes the external address.
An EASS server is required to manage the sharing of the external
addresses, i.e., to allocate and de-allocate external addresses to
the machines which do not have permanent external addresses. This
service can be provided by using the "Dynamic Host Configuration
Protocol (DHCP)" .
Many hosts do an inverse lookup of incoming connections. Therefore,
it is desirable the entry in the DNS server be updated whenever a new
external address is allocated. This will also allow an machine which
currently has a temporary external address to be called by other
machines. The updating of the entry in the DNS server can be done
more easily if the EASS server and DNS server are co-located.
We would like to thank J. K. Reynolds for the network statistics, and
V. Cerf, C. Topolcic, K. McCloghrie, R. Ullmann and K. Carlberg for
their useful comments and discussion.
 Chiappa, N., "The IP Addressing Issue", work in progress,
 Clark, D., Chapin, L., Cerf, V., Braden, R., and R. Hobby,
"Towards the Future Architecture", RFC 1287, MIT, BBN, CNRI,
ISI, UC Davis, December 1991.
 Solensky, F., and F. Kastenholz, "A Revision to IP Address
Classifications", work in progress, March 1992.
 Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:
an Address Assignment and Aggregation Strategy", work in
progress, March 1992.
 Tsuchiya, P., "The IP Network Address Translator", work in
progress, March 1991.
 Droms, R., "Dynamic Host Configuration Protocol", work in
progress, March 1992.
Security issues are not discussed in this memo.
Dept. of Computer Science
University College London
London WC1E 6BT, UK
Dept. of Computer Science
University College London
London WC1E 6BT, UK