A Two-Tier Address Structure for the Internet: A Solution to the Problem of Address Space Exhaustion
RFC 1335

Document Type RFC - Informational (May 1992; No errata)
Authors Zheng Wang  , Jon Crowcroft 
Last updated 2013-03-02
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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

Wang & Crowcroft                                                [Page 1]
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
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