"You can think of a wireless router as the center of a single
LAN. Every device connected to a single router—whether
connected physically by Ethernet cable or wirelessly—is part
of the same LAN.
"Now suppose you have a device that is not part of the LAN, but
you want it to be. The problem is that you don't have an easy way
to connect it. For example, you might have a printer or a gaming
console, such as the Xbox or Wii, which only has a cabled Ethernet
connection. If the device is far away or on another floor from your
wireless router, running a cable could be complicated.
"The solution is a wireless bridge. Using a second wireless
router installed with DD-WRT, the router will connect to your
primary router and share the network with any connected Ethernet
devices (most routers have four built-in Ethernet ports, but you
can add one or more external switches with four, eight, or more
ports each to expand even further)."