Ethernet bridging is a way to connect networks together to form a larger network. The standard for bridging is ANSI/IEEE 802.1d. A bridge is a way to connect two separate network segments together in a protocol-independent way. Packets are forwarded based on Ethernet address, rather than IP address (like a router). Since forwarding is done at Layer 2, all protocols can go transparently through a bridge.
For a more simplier explaination,
A Bridge is a device that links 1 or more network segment that uses the same network technologies. One simple analogies is the typical hub where you can plug in as many boxes you wish and they become part of a single hub device. It can also be used to modify or sniff the traffic
To bridge 2 physical network interfaces, you will need root access
# brctl addbr br0
# brctl addif br0 eth0
# brctl addif br0 eth1
# echo 1 > /proc/sys/net/ipv4/ip_forward
# ifconfig eth0 up
# ifconfig eth1 up
# ifconfig br0 up
For more information on Ethernet Bridge, do read up the following article which this blog entry relies totally on
- Kernel Korner - Linux as an Ethernet Bridge (Linux Journal)
- Linux Networking Bridge by Tom Salmon
