This is the original address format that is used on TCP/IP networks. IPv4 addresses are 32 bits in length. IPv4 addresses were originally allocated to various organizations in contiguous blocks of 16777216 (Class A), 65536 (Class B), or 256 addresses (Class C) addresses. Each organization that requested an address block received a fixed address prefix and an implied prefix mask, both specified in dotted decimal notation. For example, the Internet Assigned Numbers Authority (IANA) allocated the Class A address block 156.0.0.0 netmask 255.0.0.0 to the American Registry for Internet Numbers (ARIN). All addresses whose first byte equals 156 are within this address block. ARIN sub-allocated the Class B address block 156.151.0.0 netmask 255.255.0.0 from its Class A block to Sun Microsystems (now Oracle).
Later, The Internet Engineering Task Force (IETF) developed Classless Inter-Domain Routing (CIDR) addresses as an interim remedy for the shortage of IPv4 addresses and the limited capacity of the global Internet routing tables. CIDR address allocations are subdivided on whatever bit boundary best meets an organization's requirements. Address blocks are specified as a dotted decimal IPv4 address followed by a slash and the address prefix length in bits.
For more information, refer to the following resources:
Internet Protocol DARPA Internet Program Protocol Specification
Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan
The following table provides example subnet length specifications in both CIDR notation and dotted decimal format, as well as the total number of hosts that are possible on a network with that prefix length.
|