RFC 1917 (rfc1917) - Page 2 of 10

An Appeal to the Internet Community to Return Unused IP Networks (Prefixes) to the IANA

RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996


   The initial design of IP had all addresses the same, eight bits of
   network number and twenty four bits of host number.  The expectation
   was of a few, large, global networks.  During the first spurts of
   growth, especially with the invention of LAN technologies, it became
   obvious that this assumption was wrong and the separation of the
   address space into three classes (Class A for a few huge networks;
   Class B for more, smaller networks; and Class C for those really
   small LANs, with lots of network numbers) was implemented.  Soon
   subnets were added so sites with many small LANs could appear as a
   single network to others, the first step at limiting routing table
   size.  And finally, CIDR was introduced to the network, to add even
   more flexibility to the addressing, extending the split from three
   classes to potentially thirty different classes.

   Subnets were introduced to provide a mechanism for sites to divide a
   single network number (Class A, B, or C) into pieces, allowing a
   higher utilization of address space, and thus promoting conservation
   of the IPv4 address space.  Because of the built-in notion of
   classful addresses, subnetting automatically induced a reduction in
   the routing requirements on the Internet.  Instead of using two (or
   more) class C networks, a site could subnet a single class B into two
   (or more) subnets.  Both the allocation and the advertisement of a
   route to the second and succeeding class C's are saved.

   Since 1993, the concept of classless (the "C" in CIDR) addresses have
   been introduced to the Internet community.  Addresses are
   increasingly thought of as bitwise contiguous blocks of the entire
   address space, rather than a class A,B,C network.  For example, the
   address block formerly known as a Class A network, would be referred
   to as a network with a /8 prefix, meaning the first 8 bits of the
   address define the network portion of the address.  Sometimes the /8
   will be expressed as a mask of 255.0.0.0 (in the same way a 16 bit
   subnet mask will be written as 255.255.0.0).

   This scheme allows "supernetting" of addresses together into blocks
   which can be advertised as a single routing entry.  The practical
   purpose of this effort is to allow service providers and address
   registries to delegate realistic address spaces to organizations and
   be unfettered by the traditional network classes, which were
   inappropriately sized for most organizations.  For example the block
   of 2048 class C network numbers beginning with 192.24.0.0 and ending
   with 192.31.255.0 can be referenced as 192.24/19, or 192.24.0.0 with
   a mask of 255.248.0.0 (i.e. similar to a 19 bit subnet mask written
   in dotted decimal notation).  The concept of "supernetting" allows
   the remaining Internet address space to be allocated in smaller
   blocks, thus allowing more networks and better efficiency.  For a
   more detailed discussion refer to RFC 1518.




Nesser                   Best Current Practice