Public and the Private Address
In IP terminology, a private network is typically a network that uses private IP address.
Although most of the IPV4 address are the public address designate for use in network that are accessible on the internet, there are blocks of addresses used in the network that require limited or no internet access. These addresses are called private address. The private address is allocated to the company and office and they are not globally delegated. Private IP addresses were defined in effort to delay IPV4 exhaustion but having the feature of internet protocol, IPV6 address.
The Private address blocks are-
1.10.0.0.0/8 (10.0.0.0 to 10.255.255.255)
2.172.16.0.0/12 (172.16.0.0 to 172.31.255.255)
3.192.168.0.0/16 (192.168.0.0 to 192.168.255.255)
The use of Private address need not be unique in outside network. The internal network still design network addresses to ensure that the host in the private network use IP address that are unique within their networking environment and the packet using these address as the source and destination should not appear on the public internet. Many hosts in different network can use the same private addresses. The networking device like router at the perimeter of these private network must block or translate these addresses.
The packet with the private address is not routable across the internet and many private network use the same IP addresses then a common problem occur when merging the network, the collision of addresses, resulting of duplication of address on multiple devices. In this case the services to translate packets from host using private address are required. These services are called the Network Address Translation. At the perimeter router, NAT changes the private address in the IPV4 packet header to the Public address.