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This chapter is from the book

Identifying MAC Addresses

Objective:

2.1 Identify a MAC (Media Access Control) address and its parts.

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 hexadecimal address that allows a 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.

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.

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 Organizational 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

Platform

Method

Windows 95/98/Me

Run the winipcfg utility

Windows 2000/2003/XP

Run ipconfig /all from a command prompt

Linux/some Unix

Run the ifconfig -a command

Novell NetWare

Run the config command

Cisco router

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

Figure 3.28 The output from the ipconfig /all command on a Windows 2000 system.

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