Computer networks Part 1
Jun 18, 2009 11:16 AM, By Russell Brown
Figure 2: MAC addressing at work with a network router
Click to enlarge
Switches route data via a device’s MAC address, which is a unique address burned into every network card or device that attaches to the network. When data is sent out from a device and to the network switch, it carries with it the IP and MAC address of the sender and the IP and MAC address of the intended recipient. The network switch looks at the MAC address and routes the data accordingly. (See Figure 2.)
The network switch knows which of its ports is connected to which MAC addressed device because when each device is first connected to the network switch, the switch asks for the device’s MAC address and stores it in a table for later use. The sending device knows what MAC address to send out with the IP address because if it does not have the MAC address in its own Address Resolution Protocol (ARP) cache, then it sends out a request over the network to all devices asking for the MAC address of the owner of this particular IP address. When the device responds again, this data is stored on the sending device as a table (ARP cache).
If the receiving device has been changed out with a new one that has the same IP but different MAC address, then the message sent out has the correct IP but incorrect MAC address; the network switch cannot route it and there is no reply. In this case, the sending device makes another request asking for the MAC address associated with this IP address. Using MAC addresses allows the network device to be addressed and data routed through a switch even before it has an IP address. DHCP is used to find and address a device to assign it a dynamic IP address. Because the switch remembers which port is connected to which MAC address and directs data traffic in an orderly way, this reduces overall network traffic.
There are also managed switches that can perform complex filtering and other functions. These, however, require setup before use and knowledge of how to configure them. One of the advantages of managed switches is their ability to direct all traffic to one of their ports so a computer can analysis the data; this is called port mirroring. This could be done for traffic load analysis to see who is using most of the network’s bandwidth, for example. Another way to gain this type of access is to use a network hub between the switch and the Internet gateway. You won’t see all of your network’s traffic, but you will see all traffic going to and coming from the Internet.
Next time
Put all the pieces together and see a network in action.
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