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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 single class A
20-bit block 16 contiguous class Bs
16-bit block 256 contiguous class Cs

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

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