rfc1335.txt

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


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


   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.

Acknowledgements

   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.

References

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


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: [e-mail]


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

   EMail: [e-mail]































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