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A Two-Tier Address Structure for the Internet: A Solution to the Problem of Address Space Exhaustion :: RFC1335

Network Working Group                                          Z. Wang
Request for Comments: 1335                                J. Crowcroft
                                             University College London
                                                              May 1992

             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

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RFC 1335      Two-Tier Address Structure for the Internet       May 1992

   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 Scheme

   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

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   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
      inter-network links.

   *  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
      servers, etc.

   *  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

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      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 Advantages

   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

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      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
      be interrupted.

The Modifications

   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)" [6].

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   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.


   [1]  Chiappa, N., "The IP Addressing Issue", work in progress,
        October 1990.

   [2]  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.

   [3]  Solensky, F., and F. Kastenholz, "A Revision to IP Address
        Classifications", work in progress, March 1992.

   [4]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:
        an Address Assignment and Aggregation Strategy", work in
        progress, March 1992.

   [5]  Tsuchiya, P., "The IP Network Address Translator", work in
        progress, March 1991.

   [6]  Droms, R., "Dynamic Host Configuration Protocol", work in
        progress, March 1992.

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Security Considerations

   Security issues are not discussed in this memo.

Authors' Addresses

   Zheng Wang
   Dept. of Computer Science
   University College London
   London WC1E 6BT, UK

   EMail: z.wang@cs.ucl.ac.uk

   Jon Crowcroft
   Dept. of Computer Science
   University College London
   London WC1E 6BT, UK

   EMail: j.crowcroft@cs.ucl.ac.uk

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