Routing Between Segments

Routers, unlike bridges or switches, have the capability to operate at the first three layers of the OSI reference model---the physical, data link, and network layers. Consequently, they aren't as limited in their segmentation capabilities as bridges and switches are. They can interconnect two or more LANs without consolidating their MAC broadcast domains! In fact, using a router to segment a LAN creates fully separate LANs, each with its own media access and MAC broadcast domains. Figure 3-13 illustrates a router being used to segment a LAN.

Figure 3-13: Routers can segment both media access and MAC broadcast domains.

Figure 3-13: Routers can segment both media access and MAC broadcast domains.

In Figure 3-13, two Ethernet LANs are interconnected via a router. Each LAN's media access domain now includes the hub port and router port that provide the interconnection. The two LANs' MAC broadcast domains, however, remain fully separate.

Commonality between these LANs is established at the network layer. In other words, a Layer 3 addressing architecture and protocol suite, such as IP, is required for communications between any two devices that reside on different LANs. Given this, a third domain must be considered whenever segmenting a LAN: the network domain. A network domain consists of all the networked devices that can communicate directly using IP (or other Layer 3 protocols) for addressing across a LAN. Implicit in this definition is that IP packets are not routed to other networks, even though they use a routable address format. Routers are unique in their capability to segment network domains.

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