Bridging
From DD-WRT Wiki
Bridging
A router using DD-WRT v24 is able to create wireless bridges. The type of bridging mode to be used is selected using a drop down menu on the Wireless > Basic Settings page.
There are several variations of bridging. Use the following to determine which type is best for you. See the linked tutorials for information and examples of the different bridge types.
• Client Mode Wireless - In this type of bridge, your DD-WRT router connects (using radio) to an AP (access point) or wireless router. The DD-WRT router creates a subnet LAN. The subnet LAN has clients connected to it with cable connections.
• Client Bridge - In this bridge, your DD-WRT router connects to an AP or wireless router and extends the original LAN. No subnet is created. Wired clients connect to your secondary router and both routers are in the same LAN.
If you don't require to extend the range your wireless LAN, this is probably your best option. An example of this would be to hook up one or two computers in a bedroom where there is no wired connection available.
One advantage of the client bridge is that secondary computers can obtain DHCP leases from the main router. You also don't need to setup port forwarding on the secondary router. Security measures or access limits can be controlled from the main router.
• Repeater Bridge - This bridge is like a client bridge, except it adds the functionality of extending the range of your wireless network. The range has been extended because (presumably) the secondary router is physically some distance removed from the first. In repeater bridge mode, a wireless computer can connect to either your main router or the secondary router (repeater bridge router).
Unless you need to extend your wireless coverage, it's probably not desirable to use this mode. Some of the radio bandwidth will be consumed by the repeater function.
Other modes not discussed here are AP, AdHoc and Repeater. Although these modes utilize the router radio, they are not considered to be bridges.