Private network - Definition and Overview

In Internet terminology, a private network is a network that uses RFC 1918 IP address space rather than publicly routed IP space. If implemented according to specifications, the network is either not connected to the Internet, or there is a NAT device between the private network and the rest of the Internet. The NAT device will have at least one "real" or publicly-reachable address, and will route traffic destined for the public Internet through that address.

Private networks are becoming quite common in office LAN designs, as many organizations do not see a need for globally unique IP addresses for every computer/printer/etc that the organization uses.

This can cause problems, however, when organizations try to connect two networks that both use private address spaces. There is the potential for clashes and routing problems if both networks use the same IPs for their private networks, or rely on NAT to connect them through the Internet.