2.2.5.1 Classical IP Addressing Architecture

Although well documented elsewhere [INTERNET:2], it is useful to describe the historical use of the network prefix. The language developed to describe it is used in this and other documents and permeates the thinking behind many protocols.

The simplest classical network prefix is the Class A, B, C, D, or E network prefix. These address ranges are discriminated by observing the values of the most significant bits of the address, and break the address into simple prefix and host number fields. This is described in [INTERNET:18]. In short, the classification is:

This simple notion has been extended by the concept of subnets. These were introduced to allow arbitrary complexity of interconnected LAN structures within an organization, while insulating the Internet system against explosive growth in assigned network prefixes and routing complexity. Subnets provide a multi-level hierarchical routing structure for the Internet system. The subnet extension, described in [INTERNET:2], is a required part of the Internet architecture. The basic idea is to partition the <Host-number> field into two parts: a subnet number, and a true host number on that subnet:

      IP-address ::=
        { <Network-number>, <Subnet-number>, <Host-number> }

The interconnected physical networks within an organization use the same network prefix but different subnet numbers. The distinction between the subnets of such a subnetted network is not normally visible outside of that network. Thus, routing in the rest of the Internet uses only the <Network-prefix> part of the IP destination address. Routers outside the network treat <Network-prefix> and <Host-number> together as an uninterpreted rest part of the 32-bit IP address. Within the subnetted network, the routers use the extended network prefix:

      { <Network-number>, <Subnet-number> }

The bit positions containing this extended network number have historically been indicated by a 32-bit mask called the subnet mask. The <Subnet-number> bits SHOULD be contiguous and fall between the <Network-number> and the <Host-number> fields. More up to date protocols do not refer to a subnet mask, but to a prefix length; the "prefix" portion of an address is that which would be selected by a subnet mask whose most significant bits are all ones and the rest are zeroes. The length of the prefix equals the number of ones in the subnet mask. This document assumes that all subnet masks are expressible as prefix lengths.

The inventors of the subnet mechanism presumed that each piece of an organization's network would have only a single subnet number. In practice, it has often proven necessary or useful to have several subnets share a single physical cable. For this reason, routers should be capable of configuring multiple subnets on the same physical interfaces, and treat them (from a routing or forwarding perspective) as though they were distinct physical interfaces.