RFC 1917 (rfc1917) - Page 2 of 10
An Appeal to the Internet Community to Return Unused IP Networks (Prefixes) to the IANA
Alternative Format: Original Text Document
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