A bridge is a device that separates two or more network segments
within one logical network (e.g. a single IP-subnet).
A bridge is usually placed between two separate groups of computers
that talk with each other, but not that much with the computers in
the other group.
A good example of this is to consider a cluster of Macintoshes and a
cluster of Unix machines.
Both of these groups of machines tend to be quite chatty amongst
themselves, and the traffic they produce on the network causes
collisions for the other machines who are trying to speak to one
another.
The job of the bridge is to examine the destination of the
data packets one at a time and decide whether or not to pass the
packets to the other side of the Ethernet segment.
The result is a faster, quieter network with less collisions.
The bridging code decides whether to bridge data or to drop it not
by looking at the protocol type (IP, IPX, NetBEUI), but by looking at
the MAC-address unique to each NIC.
Important: It's vital to understand that a bridge is neither a router nor
a fire-wall.
Spoken in simple term a bridge behaves like a network switch
(i.e. Layer 2 Switch), making it a transparent network component
(which is not absolutely true, but nearly).
Read more about this at Section 4.
In addition, you can overcome hardware incompatibilities with a
bridge, without leaving the address-range of your IP-net or subnet.
E.g. it's possible to bridge between different physical media like
10 Base T and 100 Base TX.
My personal reason for starting to set up a bridge was that in my
work I had to connect Fast Ethernet components to a existing HP
Voice Grade network, which is a proprietary networking standard.
Features Above Pure Bridging
- STP
The Spanning Tree Protocol is a nifty method of keeping
Ethernet devices connected in multiple paths working.
The participating switches negotiate the shortest available path
by STP.
This feature will be discussed in
Section 7.1.
- Multiple Bridge Instances
Multiple bridge instances allow you to have more than one
bridge on your box up and running, and to control each instance
separately.
- Fire-walling
There is a patch to the bridging code which allows you
to use IP chains on the interface inside a bridge.
More info about this you'll find at
Section 7.2.
Take our 
