[Jimbo]

OK thanks to all.
I have downloaded the linksys WRT54G manual and the backplane has 5 ethernet ports, quote

Internet
The Internet port is where you connect your cable or
DSL modem through an Ethernet network cable.
Your modem connection will not work from any
other port.

Ports 1-4
These four LAN (Local Area Network) ports con-nect
to network devices, such as PCs, print servers,
and network attached storage (NAS). Each of the
ports will auto-detect whether a straight-through or
crossover cable is plugged into it, so there is no need
for an uplink port. Any one of the four ports can
serve as an uplink port to other network devices.

Jimbo