Classless Inter-Domain Routing
An IP address is interpreted as composed of two parts: a network-identifying prefix followed by a host identifier within that network. In the previous classful network architecture, IP address allocations were based on the bit boundaries of the four octets of an IP address. An address was considered to be the combination of an 8, 16, or 24-bit network prefix along with a 24, 16, or 8-bit host identifier respectively. Thus, the smallest allocation and routing block contained only 256 addresses—too small for most enterprises, and the next larger block contained addresses—too large to be used efficiently even by large organizations. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation.
List of IP protocol numbers
Below are the Internet Protocol numbers found in the "Protocol" field of the IPv4 header and the "Next Header" field of the IPv6 header. The same values are used in both versions of the field and define the layout of the header that will immediately follow the IPv4 or IPv6 header. Both the "Protocol" and "Next Header" fields are eight bits wide. The numbers used in these fields are managed and assigned by the Internet Assigned Numbers Authority (IANA).