In the early stages of development of the Internet Protocol,[1] network administrators interpreted an IP address in two parts, network number portion and host number portion. The highest order octet (most significant eight bits) in an address was designated the network number and the rest of the bits were called the rest field or host identifier and were used for host numbering within a network. This method soon proved inadequate as additional networks developed that were independent from the existing networks already designated by a network number. In 1981, the Internet addressing specification was revised with the introduction of classful network architecture.[2]
Classful network design allowed for a larger number of individual network assignments. The first three bits of the most significant octet of an IP address was defined as the class of the address. Three classes (A, B, and C) were defined for universal unicast addressing. Depending on the class derived, the network identification was based on octet boundary segments of the entire address. Each class used successively additional octets in the network identifier, thus reducing the possible number of hosts in the higher order classes (B and C). The following table gives an overview of this now obsolete system.
| Historical classful network architecture | ||||||
|---|---|---|---|---|---|---|
| Class | First octet in binary | Range of first octet | Network ID | Host ID | Number of networks | Number of addresses |
| A | 0XXXXXXX | 0 - 127 | a | b.c.d | 27 = 128 | 224 = 16,777,216 |
| B | 10XXXXXX | 128 - 191 | a.b | c.d | 214 = 16,384 | 216 = 65,536 |
| C | 110XXXXX | 192 - 223 | a.b.c | d | 221 = 2,097,152 | 28 = 256 |
The articles 'subnetwork' and 'classful network' explain the details of this design.
Although classful network design was a successful developmental stage, it proved unscalable in the rapid expansion of the Internet and was abandoned when Classless Inter-Domain Routing (CIDR) was created for the allocation of IP address blocks and new rules of routing protocol packets using IPv4 addresses. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing on arbitrary-length prefixes.
Today, remnants of classful network concepts function only in a limited scope as the default configuration parameters of some network software and hardware components (e.g. netmask), and in the technical jargon used in network administrators' discussions.
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