Classful network
Encyclopedia
A classful network is a network addressing architecture used in the Internet
from 1981 until the introduction of Classless Inter-Domain Routing
in 1993. The method divides the address
space for Internet Protocol Version 4 (IPv4
) into five address classes. Each class, coded in the first four bits of the address, defines either a different network size, i.e. number of hosts
for unicast
addresses (classes A, B, C), or a multicast
network (class D). The fifth class (E) address range is reserved for future or experimental purposes.
Since its discontinuation, remnants of classful network concepts remain in practice only in limited scope in the default configuration parameters of some network software and hardware components (e.g., default subnet mask), but the terms are often still used in general discussions of network structure among network administrators.
which was assigned the network number 10, and before the wide proliferation of local area network
s (LANs). As a consequence of this architecture, the address space supported only a low number (254) of independent networks, and it became clear very early on that this would not be enough.
(IP) packet structure, and avoid the renumbering of the existing networks. The solution was to expand the definition of the network number field to include more bits, allowing more networks to be designated, each potentially having fewer hosts. All existing network numbers at the time were smaller than 64, they only used the 6 least-significant bits of the network number field. Thus it was possible to use the most-significant bits of an address to introduce a set of address classes, while preserving the existing network numbers in the first of these classes.
The new addressing architecture was introduced by RFC 791 in 1981 as a part of the specification of the Internet Protocol. It divided the address space into primarily three address formats, henceforth called address classes, and left a fourth range reserved to be defined later.
The first class, designated as Class A, contained all addresses in which the most significant bit is zero. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including the zero network, and including the existing IP networks already allocated. A Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0. For these networks, the network address was given by the next 14 bits of the address, thus leaving 16 bits for numbering host on the network for a total of addresses per network. Class C was defined with the 3 high-order bits set to 1, 1, and 0, and designating the next 21 bits to number the networks, leaving each network with 256 local addresses.
The leading bit sequence 111 designated an "escape to extended addressing mode", which was later subdivided in to Class D (1110) for multicast addressing, while leaving as reserved for future use the 1111 block designated as Class E.
This addressing scheme is illustrated in the following table:
The number of addresses usable for addressing specific hosts in each network is always 2N - 2 (where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host portion for network address and the all-bits-one host portion as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the number of hosts is 254.
Today, IP addresses are associated with a subnet mask. This was not required in a classful network because the mask was implicitly derived from the IP address itself. Any network device would inspect the first few bits of the IP address to determine the class of the address.
(CIDR), starting in 1993 with the specification of RFC 1518 and RFC 1519, to attempt to solve this problem.
Early allocations of IP addresses by the Internet Assigned Numbers Authority
(IANA) were in some cases not made efficiently, which contributed to the problem. However, the commonly held notion that some American organizations unfairly or unnecessarily received Class A networks is wrong; most such allocations date to the period before the introduction of address classes, when the only address blocks available were what later became known as Class A networks.
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
from 1981 until the introduction of Classless Inter-Domain Routing
Classless Inter-Domain Routing
Classless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...
in 1993. The method divides the address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
space for Internet Protocol Version 4 (IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
) into five address classes. Each class, coded in the first four bits of the address, defines either a different network size, i.e. number of hosts
Host (network)
A network host is a computer connected to a computer network. A network host may offer information resources, services, and applications to users or other nodes on the network. A network host is a network node that is assigned a network layer host address....
for unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
addresses (classes A, B, C), or a multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
network (class D). The fifth class (E) address range is reserved for future or experimental purposes.
Since its discontinuation, remnants of classful network concepts remain in practice only in limited scope in the default configuration parameters of some network software and hardware components (e.g., default subnet mask), but the terms are often still used in general discussions of network structure among network administrators.
Background
Originally, a 32-bit IPv4 address was logically subdivided into the network number field, the most-significant 8 bits of an address, which specified the particular network a host was attached to, and the local address, also called rest field (the rest of the address), which uniquely identifies a host connected to that network. This format was sufficient at a time when only a few large networks existed, such as the ARPANETARPANET
The Advanced Research Projects Agency Network , was the world's first operational packet switching network and the core network of a set that came to compose the global Internet...
which was assigned the network number 10, and before the wide proliferation of local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LANs). As a consequence of this architecture, the address space supported only a low number (254) of independent networks, and it became clear very early on that this would not be enough.
Introduction of address classes
Expansion of the network had to ensure compatibility with the existing address space and the Internet ProtocolInternet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) packet structure, and avoid the renumbering of the existing networks. The solution was to expand the definition of the network number field to include more bits, allowing more networks to be designated, each potentially having fewer hosts. All existing network numbers at the time were smaller than 64, they only used the 6 least-significant bits of the network number field. Thus it was possible to use the most-significant bits of an address to introduce a set of address classes, while preserving the existing network numbers in the first of these classes.
The new addressing architecture was introduced by RFC 791 in 1981 as a part of the specification of the Internet Protocol. It divided the address space into primarily three address formats, henceforth called address classes, and left a fourth range reserved to be defined later.
The first class, designated as Class A, contained all addresses in which the most significant bit is zero. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including the zero network, and including the existing IP networks already allocated. A Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0. For these networks, the network address was given by the next 14 bits of the address, thus leaving 16 bits for numbering host on the network for a total of addresses per network. Class C was defined with the 3 high-order bits set to 1, 1, and 0, and designating the next 21 bits to number the networks, leaving each network with 256 local addresses.
The leading bit sequence 111 designated an "escape to extended addressing mode", which was later subdivided in to Class D (1110) for multicast addressing, while leaving as reserved for future use the 1111 block designated as Class E.
This addressing scheme is illustrated in the following table:
| Class | Leading bits |
Size of network number bit field |
Size of rest bit field |
Number of networks |
Addresses per network |
Start address | End address |
|---|---|---|---|---|---|---|---|
| Class A | 0 | 8 | 24 | 128 (27) | 16,777,216 (224) | 0.0.0.0 | 127.255.255.255 |
| Class B | 10 | 16 | 16 | 16,384 (214) | 65,536 (216) | 128.0.0.0 | 191.255.255.255 |
| Class C | 110 | 24 | 8 | 2,097,152 (221) | 256 (28) | 192.0.0.0 | 223.255.255.255 |
| Class D (multicast Multicast In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires... ) |
1110 | not defined | not defined | not defined | not defined | 224.0.0.0 | 239.255.255.255 |
| Class E (reserved) | 1111 | not defined | not defined | not defined | not defined | 240.0.0.0 | 255.255.255.255 |
The number of addresses usable for addressing specific hosts in each network is always 2N - 2 (where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host portion for network address and the all-bits-one host portion as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the number of hosts is 254.
Today, IP addresses are associated with a subnet mask. This was not required in a classful network because the mask was implicitly derived from the IP address itself. Any network device would inspect the first few bits of the IP address to determine the class of the address.
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
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
The replacement of classes
The first architecture change extended the addressing capability in the Internet, but did not prevent IP address shortage. The problem was that many sites needed larger address blocks than a Class C network provided, and therefore they received a Class B block, which was in most cases much larger than required. In the rapid growth of the Internet, the pool of unassigned Class B addresses (214, or about 16,000) was rapidly being depleted. Classful networking was replaced by Classless Inter-Domain RoutingClassless Inter-Domain Routing
Classless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...
(CIDR), starting in 1993 with the specification of RFC 1518 and RFC 1519, to attempt to solve this problem.
Early allocations of IP addresses by the Internet Assigned Numbers Authority
Internet Assigned Numbers Authority
The Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...
(IANA) were in some cases not made efficiently, which contributed to the problem. However, the commonly held notion that some American organizations unfairly or unnecessarily received Class A networks is wrong; most such allocations date to the period before the introduction of address classes, when the only address blocks available were what later became known as Class A networks.
See also
- IPv4 subnetting referenceIPv4 subnetting referenceIn the IPv4 address space certain address blocks are specially allocated or reserved for special uses such as loopback interfaces, private networks , and state-less autoconfiguration of interfaces. Such addresses may be used without registration or allocation from Regional Internet Registries...
- Classless Inter-Domain RoutingClassless Inter-Domain RoutingClassless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...
- List of assigned /8 IPv4 address blocks
- SubnetworkSubnetworkA subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....
- Private networkPrivate networkIn the Internet addressing architecture, a private network is a network that uses private IP address space, following the standards set by RFC 1918 and RFC 4193. These addresses are commonly used for home, office, and enterprise local area networks , when globally routable addresses are not...
- common use of classful networks