The two previous chapters discussed routers, which operate between networks. An internetwork is, by definition, a collection of local area networks connected by routers. In other words, a packet that has to traverse one or more routers has traveled across an internetwork. Eventually, that packet will arrive at the destination network-home of the destination IP address.
But what then? At that juncture, the message has gotten past the last router and must worm its way through the destination network's wiring. In other words, it must drop out of the internetwork cloud and look not for yet another router interface, but for the specific connection port into which the destination host is plugged. Since most PCs and servers do not have network connections directly to a router, there is one last leg of the journey.
So, hosts don't have their own router ports. Well, hosts-PCs and servers, for example-need to hook up to networks somehow. The router is designed to connect networks to other networks, so it is of little use when it comes to physically connecting hosts. That's where switches come in. Switches provide local connectivity to hosts, and are the building blocks with which LANs are pieced together.
The last leg of a message's journey takes place inside a building or within an office campus. Here, the transition must be made from the internetworking cloud's telecom lines, down into the cabling strung through the walls and ceilings of the building, all the way out to a wall plate, and finally to the host device itself. This final stage requires making the transition to the destination host's physical address. This address is called the media access control address, or MAC. The IP address gets you to the neighborhood, while the MAC address gets you to the front door of the house. MAC addresses should always be unique, and they identify the actual NIC (network interface card) connecting the destination device to its LAN. They serve as a kind of serial number for a physical device, so any device on any network in the world can be uniquely identified.
But a one-step shift from a worldwide IP address down to an individual host's MAC address would be too abrupt. There needs to be an intermediary step separating the high-speed router level from slow-speed NICs. Having no buffer zone would, in effect, put side-street traffic onto the interstate highway. Even if NICs and hosts were lightningfast, a middle level would still be necessary just to make things manageable. A switch provides that intermediary step. Figure 5-1 shows where they fit in.
Figure 5-1: Switches mediate between backbones and hosts
In this chapter, we delve into that zone sitting between the desktop and the router that links it to the LAN. This is the realm of cables and connectors. We realize eyes tend to glaze over when talk turns to cable plants and patch panels, as if these things are for some reason best left to the building janitor, but the subject of local connectivity is not as mundane as you might think. As high-tech networking equipment inches outward from the backbone toward the desktop, deciding how to connect individual hosts and workgroups has become strategic to the big picture of enterprise internetworking. Technology advances are happening quickly in the field of local connectivity. Thus, it's important that you understand the basics of this subject area, including some specifics on how Cisco switches work.