RFC 3021 (rfc3021) - Page 2 of 10

Using 31-Bit Prefixes on IPv4 Point-to-Point Links

RFC 3021             31-Bit Prefixes on IPv4 Links         December 2000


   - Deployment of a new Internet Protocol to increase the size of the
     address space.  One such protocol, IPv6 [RFC 2460], has been through
     the IETF process but has yet to see production deployment.  Should
     it be, deployed, it will still face a many year transition period.

   Prior to the availability of a larger address space, it seems prudent
   to consider opportunities for making more efficient use of the
   existing address space.

   One such (small) opportunity is to change the way that point-to-point
   links are numbered.  One option, which is used today on some parts of
   the Internet, is to simply not number point-to-point links between
   routers.  While this practice may seem, at first, to handily resolve
   the problem, it causes a number of problems of its own, including the
   inability to consistently manage the unnumbered link or reach a
   router through it, difficulty in management and debugging of those
   links, and the lack of standardization [RFC 1812].

   In current practice, numbered Internet subnets do not use longer than
   a 30-bit subnet mask (in most cases), which requires four addresses
   per link - two host addresses, one all-zeros network, and one all-
   ones broadcast.  This is unfortunate for point-to-point links, since
   they can only possibly have two identifying endpoints and don't
   support the notion of broadcast - any packet which is transmitted by
   one end of a link is always received by the other.

   A third option is to use host addresses on both ends of a point-to-
   point link.  This option provides the same address space savings as
   using a 31-bit subnet mask, but may only be used in links using PPP
   encapsulation [RFC 1332].  The use of host addresses allows for the
   assignment of IP addresses belonging to different networks at each
   side of the link, causing link and network management not to be
   straight forward.

   This document is based on the idea that conserving IP addresses on
   point-to-point links (using longer than a 30-bit subnet mask) while
   maintaining manageability and standard interaction is possible.
   Existing documentation [RFC 950] has already hinted at the possible
   use of a 1-bit wide host-number field.

   The savings in address space resulting from this change is easily
   seen--each point-to-point link in a large network would consume two
   addresses instead of four.  In a network with 500 point-to-point
   links, for example, this practice would amount to a savings of 1000
   addresses (the equivalent of four class C address spaces).






Retana, et al.              Standards Track