Chapter Summary

At the heart of LAN operation is the MAC address. The MAC address is the unique network adapter serial number distinguishing that network card from all others on the network. The MAC address is made up of two parts: the OUI and the vendor-assigned serial number.

Half-duplex Ethernet uses CSMA/CD as the LAN access method. When an Ethernet device wants to gain access to the network, it checks to see whether the network is quiet; if the network is not quiet, the device waits a random amount of time before retrying. If the network is quiet and two devices access the line at exactly the same time, their signals collide. When the collision is detected, they both back off and each waits a random amount of time before retrying

Table 5-3 lists LAN hardware and the layer in the OSI model at which each piece of hardware operates.

Table 5-3. LAN Hardware


OSI Layer


Layer 1 (physical)


Layer 1 (physical)


Layer 2 (data link)


Layer 2 (data link) or Layer 3 (network)


Layer 3 (network)

Repeaters regenerate signals in the cable line and are used in both local-and wide-area networking environments to extend the distance a signal can reach. Ethernet hubs are multiport repeaters because each signal that is received by the hub is repeated out all hub ports and is received by any device connected to the hub.

Ethernet bridges are essentially multiport hubs. Instead of repeating the incoming signal out all ports, however, the bridge maps the MAC address to a port. This map keeps track of the MAC addresses of each node that resides on each network segment and allows only necessary traffic to pass through the bridge, such as traffic destined for a segment other than the source. If the frame's source and destination network segments are the same, the frame is filtered; if the segments differ, the frame is forwarded by the bridge to the appropriate segment.

A LAN switch is a network device that cross-connects stations or LAN segments. Network switches replace shared media hubs, increasing network bandwidth. Each port on the switch can give full bandwidth to a single server or client station or each can be connected to a hub with several stations. The switch also forwards a frame out all ports if the destination MAC address is unknown.

Routers are basically computers with two or more NICs supporting one or more network protocols, such as the Internet Protocol (IP). A switch receives frames and makes filtering and forwarding decisions based on the hardware MAC address, whereas the router opens these frames and examines the packets contained therein. The router looks at the destination network address in these packets and makes a forwarding decision based on this address. If the router does not know how to reach the destination network, the packet is dropped. The router then forwards the packet to the appropriate LAN or WAN network segment.