The ZyXEL Prestige series of routers uses what it refers to as "Single User Access" to allow a network to access the Internet through an ISP that provides a single IP address.
This uses the facility known as NAT (Network Address Translation), sometimes also called IP masquerading. NAT is really RFC 1631 (an Internet standard). The technical details are explained here.
NAT has been implemented in a small number of routers (apart from the Prestige), and in some software packages for various operating systems:
There is also quite a good page on the subject of Connecting a LAN to the Internet, which concentrates on ISDN, IP masquerading, etc.Words and design by: