Reserved Addresses

There are a few reserved IPv4 address spaces which cannot be used on the internet.
These addresses serve special purpose and cannot be routed outside the Local Area
Network.
Private IP Addresses
Every class of IP, (A, B & C) has some addresses reserved as Private IP addresses.
These IPs can be used within a network, campus, company and are private to it. These
addresses cannot be routed on the Internet, so packets containing these private
addresses are dropped by the Routers.

In order to communicate with the outside world, these IP addresses must have to be
translated to some public IP addresses using NAT process, or Web Proxy server can be
used.
The sole purpose to create a separate range of private addresses is to control
assignment of already-limited IPv4 address pool. By using a private address range within
LAN, the requirement of IPv4 addresses has globally decreased significantly. It has also
helped delaying the IPv4 address exhaustion.
IP class, while using private address range, can be chosen as per the size and
requirement of the organization. Larger organizations may choose class A private IP
address range where smaller organizations may opt for class C. These IP addresses can
be further sub-netted and assigned to departments within an organization.