Identifying MAC Addresses
Given an example, identify a Media Access Control (MAC) address.
This chapter many times refers to MAC addresses and how certain devices use them. However, it has not yet discussed why MAC addresses exist, how they are assigned, and what they consist of. Let's do that now.
A MAC address is a 6-byte address that allows an NIC to be uniquely identified on the network. The MAC address forms the basis of network communication, regardless of the protocol used to achieve network connection. Because the MAC address is so fundamental to network communication, mechanisms are in place to ensure that there is no possibility of duplicate addresses being used.
A MAC Address Is the Physical Address
A MAC address is sometimes referred to as a physical address because it is physically embedded in the interface. Sometimes it is also referred to as a network address, which is incorrect. A network address is the logical protocol address assigned to the network to which the interface is connected.
To combat the possibility of duplicate MAC addresses being assigned, the Institute of Electrical and Electronics Engineers (IEEE) took over the assignment of MAC addresses. But rather than be burdened with assigning individual addresses, the IEEE instead decided to assign each manufacturer an ID and then let the manufacturer further allocate IDs. The result is that in a MAC address, the first three bytes define the manufacturer and the last three are assigned by the manufacturer.
Because MAC addresses are expressed in hexadecimal, only the numbers 0 through 9 and the letters A through F can be used in them. If you get a Network+ exam question about identifying a MAC address and some of the answers contain letters and numbers other than 0 through 9 and the letters A through F, you can discount those answers immediately.
For example, consider the MAC address of the computer on which this book is being written: 00:D0:59:09:07:51. The first three bytes (00:D0:59) identify the manufacturer of the card; because only this manufacturer can use this address, it is known as the organizationally unique identifier (OUI). The last three bytes (09:07:51) are then referred to as the Universal LAN MAC address: They make this interface unique. You can find a complete listing of organizational MAC address assignments at http://standards.ieee.org/regauth/oui/oui.txt.
You can discover the MAC address of the NIC in various ways, depending on what system or platform you are working on. Table 3.4 defines various platforms and the method you can use to view the MAC address of an interface.
Table 3.4 METHODS OF VIEWING THE MAC ADDRESSES OF NICS
Run the winipcfg utility
Run ipconfig /all from a command prompt
Run the ifconfig -a command
Run the config command
Run the sh int <interface name> command
Figure 3.28 shows the ipconfig /all command run on a Windows 2000 system. The MAC address is defined on the Physical Address line of the output.
Figure 3.28 The output from the ipconfig /all command on a Windows 2000 system.