Classful networking is the name given to the first round of changes to the structure of the IP address in IPv4. An IP address (Internet Protocol address) is a unique address that devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ...

Classful networking is obsolete on the modern Internet. There is no longer any such thing as a class A/B/C network. The correct modern representation for what would have been referred to as a "Class B" prior to 1993 would be "a set of /16 addresses", under the Classless Inter-Domain Routing (CIDR) system. Classless Inter-Domain Routing (CIDR, pronounced cider or cedar), introduced starting in 1993, is the latest refinement to the way IP addresses are interpreted. ...

Before classes

The prototype Internet in 1982; note that all the networks (the ovals) have addresses which are single integers; the rectangles are switch.

Originally, the 32-bit IPv4 address consisted simply of an 8-bit network number field (which specified the particular network a host was attached to), and a rest field, which gave the address of the host within that network. This format was picked before the advent of local area networks (LANs), when there were only a few, large, networks such as the ARPANET. Image File history File links Download high resolution version (832x1093, 144 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): History of the Internet Classful network ... Image File history File links Download high resolution version (832x1093, 144 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): History of the Internet Classful network ... 1982 (MCMLXXXII) was a common year starting on Friday of the Gregorian calendar. ... Electrical switches. ... Local area network scheme A local area network (LAN) is a computer network covering a local area, like a home, office, or group of buildings[1]. Current LANs are most likely to be based on switched IEEE 802. ... ARPANET logical map, March 1977. ...

This resulted in a very low count (256) of network numbers being available, and very early on, as LANs started to appear, it became obvious that that would not be enough.

Classes

As a kludge, the definition of IP addresses was changed in 1981 by RFC 791 to allow three different sizes of the network number field (and the associated rest field), as specified in the table below: A kludge (or kluge) is a clumsy or inelegant solution to a problem. ...

This allowed the following population of network numbers (note that the actual numbers are slightly smaller because addresses consisting of all zeros and all ones are not allowed): Routing Schemes anycast broadcast multicast unicast Multicast is sometimes also used to refer to a multiplexed broadcast, although that is a very different thing and should not be confused. ...

The number of valid networks and hosts available is always 2^N (where N equals the number of bits used) minus 2 (one for the all zeros address and one for the all ones address). Thus, for a class C address wherein 8 bits are available for hosts, the number of hosts is 2⁸ -2, or 256 - 2, which is 254 as shown above.

The larger network number field allowed a larger number of networks, thereby accommodating the continued growth of the Internet.

The IP address netmask (which is so commonly associated with an IP address today) was not required because the mask length was part of the IP address itself. Any network device could inspect the first few bits of a 32-bit IP address to see which class the address belonged to. The word subnetwork has two related meanings. ...

The method of comparing two IP address's physical networks did not change, however (see subnet). For each address, the network number field size and its subsequent value were determined (the rest field was ignored). The network numbers were then compared. If they matched, then the two addresses were on the same network. The word subnetwork (usually shortened to subnet) has two related meanings. ...

The replacement of classes

This first round of changes was enough to work in the short run, however; an IP address shortage still developed. The principal problem was that most sites were too big for a "class C" network number, and received a "class B" number instead. With the rapid growth of the Internet, the available pool of class B addresses (basically 2¹⁴, or about 16,000 total) was rapidly being depleted. Classful networking was replaced by Classless Inter-Domain Routing (CIDR), starting in about 1993, to solve this problem (and others). As the number of Internet-connected devices increases, the address space of the current IPv4 version of the TCP/IP protocol suite is rapidly being consumed by allocations for new devices. ... Classless Inter-Domain Routing (CIDR, pronounced cider or cedar), introduced starting in 1993, is the latest refinement to the way IP addresses are interpreted. ... 1993 (MCMXCIII) was a common year starting on Friday of the Gregorian calendar and marked the Beginning of the International Decade to Combat Racism and Racial Discrimination (1993-2003). ...

Early allocations of IP addresses by IANA were in some cases not made very efficiently, which contributed to the problem. (However, the commonly-held notion that some American organizations unfairly or unnecessarily received class A networks is a canard; most such allocations date to the period before the introduction of address classes, when the only thing available was what later became known as "class A" network number.) For other uses of IANA, see IANA (disambiguation). ... This page refers to the term canard, as used in computer jargon. ...

Useful tables

Class ranges

The address ranges used for each class are given in the following table, in the standard dotted decimal notation.

Classless Inter-Domain Routing (CIDR, pronounced cider or cedar), introduced starting in 1993, is the latest refinement to the way IP addresses are interpreted. ... The word subnetwork has two related meanings. ... Routing Schemes anycast broadcast multicast unicast Multicast is sometimes also used to refer to a multiplexed broadcast, although that is a very different thing and should not be confused. ...

Special ranges

Some addresses are reserved for special uses (RFC 3330).

It has been suggested that this article or section be merged with private network. ... Zeroconf or Zero Configuration Networking is a set of techniques that automatically create a usable IP network without configuration or special servers. ... Automatic Private IP Addressing (APIPA) is the Microsoft Windows term for fallback to the link local addresses as prescribed in RFC 3330. ... It has been suggested that this article or section be merged with private network. ... Internet Protocol version 6 (IPv6) is a network layer IP standard used by electronic devices to exchange data across a packet-switched internetwork. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... It has been suggested that this article or section be merged with private network. ... In computing, a benchmark is the result of running a computer program, a set of programs, or other operations, in order to assess the relative performance of an object, by running a number of standard tests and trials against it. ... Routing Schemes anycast broadcast multicast unicast Multicast is sometimes also used to refer to a multiplexed broadcast, although that is a very different thing and should not be confused. ...

Bit-wise representation

In the following table:

• n indicates a binary slot used for network ID.
• H indicates a binary slot used for host ID.
• X indicates a binary slot (without specified purpose).

 Class A 0. 0. 0. 0 = 00000000.00000000.00000000.00000000 127.255.255.255 = 01111111.11111111.11111111.11111111 0nnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH 127>is a loop back ip address. Class B 128. 0. 0. 0 = 10000000.00000000.00000000.00000000 191.255.255.255 = 10111111.11111111.11111111.11111111 10nnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH Class C 192. 0. 0. 0 = 11000000.00000000.00000000.00000000 223.255.255.255 = 11011111.11111111.11111111.11111111 110nnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH Class D 224. 0. 0. 0 = 11100000.00000000.00000000.00000000 239.255.255.255 = 11101111.11111111.11111111.11111111 1110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX Class E 240. 0. 0. 0 = 11110000.00000000.00000000.00000000 255.255.255.255 = 11111111.11111111.11111111.11111111 1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX 

See also

• Classless Inter-Domain Routing

Classless Inter-Domain Routing (CIDR, pronounced cider or cedar), introduced starting in 1993, is the latest refinement to the way IP addresses are interpreted. ...

External links

• Classful Online IP Subnet Calculator
• RFC 791 — defines classful networking.
• Current /8 delegations from IANA (Last updated: January 19, 2006)