Allocation in IPV4
Originally, the IP address was divided into two parts:
* Network id – first octet
* Host id – last three octets
This created an upper limit of 256 networks. As the networks began to be allocated, this was soon seen to be inadequate.
To overcome this limit, different classes of network were defined, in a system which later became known as classful networking. Five classes were created (A, B, C, D, & E), three of which (A, B, & C) had different lengths for the network field. The rest of the address field in these three classes was used to identify a host on that network, which meant that each network class had a different maximum number of hosts. Thus there were a few networks with lots of host addresses and numerous networks with only a few addresses. Class D was for multicast addresses and class E was reserved.
Around 1993, these classes were replaced with a Classless Inter-Domain Routing (CIDR) scheme, and the previous scheme was dubbed “classful”, by contrast. CIDR’s primary advantage is to allow re-division of Class A, B & C networks so that smaller (or larger) blocks of addresses may be allocated to entities (such as Internet service providers, or their customers) or local area networks.
The actual assignment of an address is not arbitrary. The fundamental principle of routing is that address encodes information about a device’s location within a network. This implies that an address assigned to one part of a network will not function in another part of the network. A hierarchical structure, created by CIDR and overseen by the Internet Assigned Numbers Authority (IANA) and its Regional Internet Registries (RIRs), manages the assignment of Internet address worldwide. Each RIR maintains a publicly searchable WHOIS database that provides information about IP address assignments; information from these databases plays a central role in numerous tools that attempt to locate IP addresses geographically.