RFC1918 IP Addresses

The IETF have released many standards around the topic of NAT. RFC1918 defines a range of IP addresses which can be used for private networks and which should not be routed over the Internet.

Class	Range
A	10.0.0.0/8
B	172.16.0.0-172.31.0.0/16
C	192.168.0.0-192.168.255.0/24

Almost every SME network in the world will use 192.168.x.0/24 and every ISP supplies their CPE with a range like this pre-set.

Many big companies will use the Class B address space.

Governments, major corporations and data centres will use the Class A address space.

Of course, this can all cause real problems after a merger, as both companies may be using the 10.0.0.0/8 address space in an incompatible manner!