# How do bridges divide a larger network into smaller broadcast domains?

I am recently brushing up my computer network knowledge, and came across a seemingly peculiar statement:

A network bridge divides a network into smaller broadcast domains.

As far as I could grasp the details, a bridge operates on the two lower layers of the ISO OSI model - the physical and the data link layers. What this means is bridge selectively forwards data link frames on the basis of the destination MAC address that can be found on each received frame.

But, the doubt arises when we consider the fact that the broadcasts are achieved using a broadcast IP address of that particular network. Now, due to the broadcast address being an entity/property of the network layer, should principally be not paid attention to by the bridge, and be flooded into all the interfaces available to it.

Then, how exactly does the bridge manage to filter the broadcast packets?

• I'm not sure if this is a computer science question. It's certainly not ontopic on Network Engineering. Community votes, please: ontopic? – Raphael Jun 19 '16 at 15:34

## 1 Answer

After quite a bit of research through YouTube and whatnot, I finally managed to find the answer.

A bridge (or a switch, both being data-link layer devices) DOES NOT divide the broadcast domains of the VLAN (the virtual LAN is formed by connecting the different physical segments using the bridge/switch).

A broadcast is sent with a special MAC address FF:FF:FF:FF:FF:FF (48 bits of 1s), of which the bridge/switch has no entry in its bridge table (and for that matter, will never have). The essential point to be noted here is that a bridge/switch follows a learning approach, in which until it is absolutely sure on which of it's port (or interface) a MAC address lies, it just forwards the frame to all the ports (except the one from which the frame has arrived). Hence, the bridge/switch, having no details of the destination MAC address of the broadcast frame, will go on to forwarding the frame to all of its ports (except one, the one from which it arrived).

What this means is that a bridge/switch cannot break the broadcast domain of the VLAN it is forming.

More related information:

https://youtu.be/reXS_e3fTAk?t=335