A broadcast domain is based on layer three where we use IPv4 addressing. Keep in mind that to communicate on the Internet we must use TCP/IP, and the IP portion is version 4. IPv6 is not too far off in the future!
A router is used to stop broadcasts, or separate broadcast domains. Which when you think about it, it does make sense.
If layer 3 broadcast traffic were to be allowed on the Internet then the Internet would crash in a heartbeat! A router knows
where to send IP traffic based on information in the routing table. For the CompTIA® Network+ exam that is what we should be
aware of. For the Cisco® CCNA we have to think about routing protocols such as RIP, IGRP, EIGRP etc.
The following exhibit has two routers with a serial link between them. The Flintstones are on one network (192.168.12.20/20)
and the Rubbles are on the second network (192.168.13.30/24).
This would represent two Broadcast domains based on layer three IP addressing, and as well two collision domains based on layer two, since router also separates collision domains. But we will not go and buy routers to separate collision domains. That is what bridges and switches are for.
Did you notice that Fred has a bigger router than Barney?
