If you answered 5 for Figure 1-4, then you are absolutely correct since each port of the Switches represent a single collision domain. If you answered more than 5 then you need to remember that a hub does not break collision domains. Similarly, Figure 1-5 has 7 collision domains.
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Now that you know how a switch works and improves a network, consider the one problem associated with a switched network. Earlier, you learned that hubs flood out all packets, even the unicast ones. A switch does not flood out unicast packets but it does flood out a broadcast packet. All hosts connected to a switched network are said to be in the same broadcast domain. All hosts connected to it will receive any broadcast sent out in this domain. While broadcasts are useful and essential for network operations, in a large switched network too many broadcasts will slow down the network. To remedy this situation, networks are broken into smaller sizes and these separate networks are interconnected using routers. Routers do not allow broadcasts to be transmitted across different networks it interconnects and hence effectively breaks up a broadcast domain. Figure 1-6 shows three switched networks interconnected by a router.
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Figure 1-6 Router in an Internetwork
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In the network shown in Figure 1-6, broadcasts from hosts connected to Switch1 will not reach hosts connected to Switch2 or Switch3. This is because the router will drop the broadcast on its receiving interface.
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In addition to breaking up broadcast domains, routers also perform the following four essential functions in your network:
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Packet Switching – At the barest minimum, routers are like switches because they essentially switch packets between networks.**
Communication between Networks – As shown in Figure 1-6, routers allow communication between networks connected to it.**
Path Selection – Routers can talk to each other to learn about all the networks connected to various routers and then select the best path to reach a network. This is function is discussed in detail later in the book.**
Packet Filtering – Routers can drop or forward packets based on certain criteria like their source and destination. This is also discussed in detail later in the book.**
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Exam Alert: Remember that switches break collision domains and routers break broadcast domains. In addition to that it is important to remember the functions of a router for your CCNA certification exam.
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Now that you know what a network is and what various network devices do, its time to learn about various network types followed by networking models.