Private IP address

Private IP address space has been allocated via RFC 1918. This means the addresses are available for any use by anyone and therefore the same RFC 1918 IP addreses can be reused. However they are defined as not routeable on the public Internet. They are used extensively in private networks due to the shortage of publicly registerable IP addresses and therefore NAT is required to connect those networks to the Internet.

Private IP addresses also provide a basic form of security as in a typical network configuation of this type it is not possible for the outside world (Internet) to establish a connection directly to a host using these addresses.

Specifically the networks are:

Name	start IP address	end IP address	classful description	largest CIDR block
24-bit block	10.0.0.0	10.255.255.255	single class A	10.0.0.0/8
20-bit block	172.16.0.0	172.31.255.255	16 contiguous class Bs	172.16.0.0/12
16-bit block	192.168.0.0	192.168.255.255	256 contiguous class Cs	192.168.0.0/16

RFC 1597 was the original specification but is now for historical purposes only.

External links

• RFC 1918