CCNA Exam Training Tutorial:

MAC Addresses, Device IDs, And The OUI

By Chris Bryant, CCIE #12933

Hexadecimal addressing, not to mention converting hex values to decimal, can be really intimidating if you haven't seen it before you study for the CCNA exam. The purpose of this Cisco training tutorial is to introduce you to hexadecimal values and allow you to become comfortable with it in order to earn your CCNA certification.

A Media Access Control (MAC) address looks like this: af-14-b3-c2-14-45

The first oddity with MAC addresses is that we've got numbers and letters in the address. Since every single networking device should have a unique MAC address, we have to use hex rather than the usual decimals we're all familiar with. Using hex to express an address allows us to have many more possible combinations than we'd have if we just used decimals.

A MAC address looks like one string of hex characters separated by dashes, but there are actually two distinct parts to this address - the OUI and the Device ID.

The Organizationally Unique Identifier (OUI) is the first half of a MAC address. An OUI gets its name from being unique to one organization; that is, the OUI af-14-b3 would be assigned to one vendor and one vendor only, such as Cisco.

The Device ID is the second half of the MAC address, and is a Device ID that the vendor has not used previously with that particular OUI. These simple rules guarantee that a MAC address will not be duplicated by a vendor. The device ID in the previous example is c2-14-45.

CCNA Exam And Real-World "Gotchas"

Some other tips and tricks to keep in mind, in both the exam room and the real world:

A hex value represented by a letter is the same whether it's an upper-case or lower-case letter. The letters "a" and "A" both represent 10, for instance.

The highest decimal value represented by a single hex character is 15, represented by the hex character "F". No valid MAC address will ever have a letter in it that comes after "F". Therefore, both of the following MAC addresses are considered invalid:

11-22-33-44-55-hf
Rf-12-34-45-56-67

You'll generally see MAC addresses expressed as you see them in the above example, but they can also be expressed using colons or even in a mock dotted-decimal formal. All of the following represent the same MAC address.

aa-bb-cc-dd-ee-34
aa:bb:cc:dd:ee:34
aabb.ccdd.ee34

We spend most of our time in networking dealing with IP addresses, but knowing how to convert hex values and how MAC addresses are created is an important skill for real-world networking as well as passing the CCNA exam. I wish you good luck in both!

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To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com