Upgrading and Repairing Networks

Note

Routers can be used to extend a network. However, the difference between a network router and a pure LAN switch is significant. Routers are used to direct network frames to the correct network or subnet on which the destination host resides, or to another router that may know of the destination network that lies in the path to the destination network.

Although routers can be used within large networked environments to segregate physical network segments, they are generally considered to be WAN devicesused to connect a LAN to the Internet or a larger private intranet. Switches, in the form most used today, are used to get past the limitations imposed by traditional LAN technologies, including the bus topology, the hub, and the bridge. However, at the end of this chapter, you'll see how switching technology has also moved up the ladder into the WAN market.

A major difference between routers and LAN switches is that a router makes decisions based on the network portion of the IP address, whereas switches work at a lower level and make decisions based on the Media Access Control (MAC) address burned into the network card by the manufacturer. Because the MAC address space is random, and the IP address space is hierarchical, LAN switches would need a routing table so large that it would not be possible to store every MAC address for every computer in the world today. Yet routers can use the network portion of an IP address to send a packet on its way because of the very nature of the hierarchical address space (network address/client address) provided by IP. LAN switches can work quickly because they only need to look at the MAC address, which is always located in the same part of the Ethernet frame that carries the IP packet as its payload.

Note that wired or wireless routers found in many SOHO networks actually contain Ethernet switches as well as routers. In essence, these devices integrate formerly-separate components into a single device.