A classful network is an obsolete network addressing architecture used in the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...

from 1981 until the introduction of

Classless Inter-Domain Routing Classless Inter-Domain Routing (CIDR ) is a method for allocating IP addresses and for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet. Its ...

(CIDR) in 1993. The method divides the

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

space for Internet Protocol version 4 (IPv4) into five address classes based on the leading four address bits. Classes A, B, and C provide

unicast Unicast is data transmission from a single sender (red) to a single receiver (green). Other devices on the network (yellow) do not participate in the communication. In computer networking, unicast is a one-to-one transmission from one point in ...

addresses for networks of three different network sizes. Class D is for

multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused wit ...

networking and the class E address range is reserved for future or experimental purposes. Since its discontinuation, remnants of classful network concepts have remained in practice only in limited scope in the default configuration parameters of some network software and hardware components, most notably in the default configuration of

subnet mask A subnetwork or subnet is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identica ...

Background

In the original address definition, the most significant eight bits of the 32-bit IPv4 address was the ''network number'' field which specified the particular network a host was attached to. The remaining 24 bits specified the local address, also called ''rest field'' (the rest of the address), which uniquely identified a host connected to that network. This format was sufficient at a time when only a few large networks existed, such as the

ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical foun ...

(network number 10), and before the wide proliferation of

local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a larger ...

s (LANs). As a consequence of this architecture, the address space supported only a low number (254) of independent networks. Before the introduction of address classes, the only address blocks available were these large blocks which later became known as Class A networks. As a result, some organizations involved in the early development of the Internet received address space allocations far larger than they would ever need (16,777,216 IP addresses each). It became clear early in the growth of the network that this would be a critical

scalability Scalability is the property of a system to handle a growing amount of work by adding resources to the system. In an economic context, a scalable business model implies that a company can increase sales given increased resources. For example, a ...

limitation.

Introduction of address classes

Expansion of the network had to ensure compatibility with the existing address space and the IPv4 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. Since all existing network numbers at the time were smaller than 64, they had 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 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 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 respectively. 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 at-the-time unspecified addressing mode ("''escape to extended addressing mode''"), which was later subdivided as Class D (''1110'') for multicast addressing, while leaving as reserved for future use the ''1111'' block designated as Class E. This architecture change extended the addressing capacity of the Internet but did not prevent IP address exhaustion. 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. Due to the rapid growth of the Internet, the pool of unassigned Class B addresses (2¹⁴, or about 16,000) was rapidly being depleted. Starting in 1993, classful networking was replaced by

(CIDR), in an attempt to solve this problem.

Classful addressing definition

Under classful network addressing, the 32-bit IPv4 address space was partitioned into 5 classes (A-E) as shown in the following tables. ;Classes ;Bit-wise representation In the following bit-wise representation, * ''n'' indicates a bit used for the network ID. * ''H'' indicates a bit used for the host ID. * ''X'' indicates a bit without a 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 number of addresses usable for addressing specific hosts in each network is always , where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host value to represent the network address and the all-bits-one host value for use as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the maximum number of hosts is 254. Today, IP addresses are associated with a

. This was not required in a classful network because the mask was implied by the address itself; any network device would inspect the first few bits of the IP address to determine the class of the address and thus its netmask. The blocks numerically at the start and end of classes A, B and C were originally reserved for special addressing or future features, i.e., and are reserved in former class A; and were reserved in former class B but are now available for assignment; and are reserved in former class C. While the network is a Class A network, it is designated for

loopback Loopback (also written loop-back) is the routing of electronic signals or digital data streams back to their source without intentional processing or modification. It is primarily a means of testing the communications infrastructure. There are m ...

and cannot be assigned to a network. Class D is reserved for

and cannot be used for regular unicast traffic. Class E is reserved and cannot be used on the public Internet. Many older routers will not accept using it in any context.

Notes

References

External links