Private IP Addressing

Most organizations have a number of computers that will never be connected to the Internet. These computers do not need globally unique IP addresses but must be unique within the organization's network. Thus, an organization could use any network number(s) it wanted, regardless of whether those network number(s) are in use on the Internet or not. However, a set of IP addresses from Class A, Class B and Class C has been set aside for use in private networks and has been defined in RFC 1918. This RFC defines a set of networks that not be assigned to any organization as a registered network number to be used on the Internet. These network numbers allow organizations to use unregistered network numbers that are not used by anyone else in the public Internet. However, no organization is allowed to advertise these networks using a routing protocol on the Internet.