Public Internet addresses must be registered by a company with an Internet authority, for example, ARIN or RIPE. These public Internet addresses can also be leased from an ISP. Private IP addresses are reserved and can be used by anyone. That means two networks, or two million networks, can each use the same private address. A router should never route RFC 1918 addresses, because ISPs typically configure the border routers to prevent privately addressed traffic from being forwarded.

NAT provides great benefits to individual companies and the Internet. Before NAT, a host with a private address could not access the Internet. Using NAT, individual companies can address some or all of their hosts with private addresses and use NAT to provide access the Internet.