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.
IP aliasing
IP aliasing can be used to provide multiple network addresses on a single physical interface. This demonstrates using IP version 4 addresses only. One reason for using this could be to make a computer look as though it is multiple computers, so for example you could have one server that is acting as both a gateway (router) and a DHCP server and DNS using 3 different IP addresses, perhaps with a future plan to use a hardware router and to move the functionality to separate DNS and DHCP servers. Or indeed the opposite you could decide to replace the 3 different hardware devices with a single server to reduce the administration overhead. In this case you can have 3 different addresses which are all on the same computer without having to install lots of physical network interfaces. Another reason is that you want to have the computer on two different logical network subnets whilst using a single physical interface.