I have been using the D-Link DI-704 with my US-Robotics 56K analog MODEM so that my wife and I can use the same phone line to connect to the Internet. The DI-704 is a four port switch connected to a NAT router that can connect to a MODEM via a 9-pin serial connection or a WAN via a RJ-45 connector.
D-Link has a DI-707 that has an seven port switch and a DI-701 that has one port. There are also wireless models. SMC and Linksys make similar units.
The NAT router function allows a number of computers to share the same IP address on the Internet. Each machine has its own unique IP on the local network.
It has a DHCP server which allows the IP addresses on the client computers to be assigned automatially, other networking parameters like DNS are acquired from the WAN and passed on to the clients automatically.
When a MODEM is used it can do dial on demand, i.e. when one of the computers on the local network attempts a connection to the Internet, a dial string is issued and the unit handles the logon with the host. When no requests occur for a settable time, the MODEM hangs up.
It works with cable MODEMs or DSL as well.
The configuration is handled via a web browser.
There are a number of simple firewall functions and a log function.