Bridges sit between segments and only pass traffic if it recognizes that the destination (MAC) address is on another segment. If it sees a MAC that it doens't know, it floods the frame out all ports except the one that sourced the frame.
Each segment operates as an independent broadcast domain, so what a switch / bridge does is break up what would be one large collision domain (like back in the "hub days") into smaller more manageble collision domains.
Switch / bridge operation is also necessary to support full duplex operation (since all hosts can have a "private" collision domain i.e., one host per port).
Check out the following link. It will cover switch / bridge operation in more detail.
This is the online version of Cisco's Inter-Networking Guide, which is also a very large (and expensive) book. If you need to find it in the future, just Google for "Internetworking" and it's always / usually at the top of the list.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...