X indicates a bit without a specified purpose.Ġ.H indicates a bit used for the host ID.n indicates a bit used for the network ID.In the following bit-wise representation, Under classful network addressing, the 32-bit IPv4 address space was partitioned into 5 classes (A-E) as shown in the following tables.ĭefault subnet mask in dot-decimal notation Starting in 1993, classful networking was replaced by Classless Inter-Domain Routing (CIDR), in an attempt to solve this problem. Due to the rapid growth of the Internet, the pool of unassigned Class B addresses (2 14, or about 16,000) was rapidly being depleted. 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. This architecture change extended the addressing capacity of the Internet but did not prevent IP address exhaustion. 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.
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. 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 65 536 addresses per network. A Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0 respectively. 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. The first class, designated as Class A, contained all addresses in which the most significant bit is zero. 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 new addressing architecture was introduced by RFC 791 in 1981 as a part of the specification of the Internet Protocol. 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. 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.
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. Introduction of address classes Įxpansion 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. It became clear early in the growth of the network that this would be a critical scalability limitation. 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!). As a consequence of this architecture, the address space supported only a low number (254) of independent networks.īefore the introduction of address classes, the only address blocks available were these large blocks which later became known as Class A networks. This format was sufficient at a time when only a few large networks existed, such as the ARPANET (network number 10), and before the wide proliferation of local area networks (LANs). 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. 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.
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