Subtitles of the Movie

Routers and layer three switches, which are basically routers, are used to create broadcast domains. A broadcast domain uses a layer three address; in other words, we communicate from one broadcast domain to another using a layer three address. A layer three address would be for example an IP address or an IPX address. Basically routers are responsible for two things, either delivering a packet to a host, if the router determines that that host is in its subnet, if it is responsible for that host, or moving the packet to the next destination, to the next router who is then responsible for delivering the packet. So a router consults a routing table to determine where to move the packet. So routers are used to control traffic and to eliminate broadcasts, but they use logical addresses, not physical addresses. At layer two we are using physical addresses, but at layer three, as you'll remember that is the network layer of the OSI model, we are now using logical addresses to make routing decisions. Now the routers build their routing tables either by the administrator typing the information in, or by routing protocols where one router talks to another router and gives its information, so that all of the routers end up with a topology of what the network looks like. And that way, they can make decisions as to which way to send a packet on its way, so that it eventually gets to the intended host. When we separate networks with layer three devices, every interface of the layer three device is a different broadcast domain. So we need to understand that a layer two device does not create broadcast domains, only collision domains. But a layer three device creates a new broadcast domain for every single one of its interfaces. That's what they are for. The layer three devices are used to transfer packets from one broadcast domain to another, so you will never have two different interfaces on the same layer three device that are on the same broadcast domain. It's important to understand that both for real life as well as for the test, and for what we are building up to. So we've contrasted broadcast domains and collision domains. In the next chapter we will discuss in more detail the types of devices that we use on a network, and their effect in creating and maintaining collision domains and broadcast domains. In particular, we are going to discuss hubs, bridges and switches. That's next. particular, we are going to discuss hubs, bridges and switches. That's next.