Visit my blog for free daily Cisco CCNA and CCNP certification questions, my latest free articles and tutorials, and more!
CCNA Exam Training Tutorial:
MAC Addresses, Device IDs, And The OUI
Now With An Exclusive Video Practice Exam!
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....
... and if you've practiced in advance, they're really easy to work with.
MAC addresses consists of hex values, and 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.
We'll take a look at some hex "gotchas" right after this brief, important message!
Some hex 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 the decimal 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.
Even though MAC addresses are a vital part of our network, we don't spend time on a daily basis figuring out which part of the address is the OUI, and we don't do a lot of hex conversion on the job.
Practice these skills just a few minutes a day, and the compound effect of that practice will result in easy points on your exam!
For plenty of additional free training videos, tutorials, and practice exams, visit my Cisco Certification Tutorial and Video page .. and be sure to scroll down for an exclusive Video Practice Exam on this subject!
