MAC Addresses--6 or 12 numbers (not characters or hex digits)

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May 2nd, 2007
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My name is Tim. I'm currently taking a review course to prep for my CCNA. This question was taken verbatim from the CCNA Guide to Cisco Networking, Third Edition. "Which of the following accurately describe the Media Access Control (MAC) address?" The instructor's answer guide shows "C. MAC addresses contain 12 hexadecimal numbers." as one of the three correct answers. I took issue with that. MAC addresses are 48 bits long (i.e. 6 hexadecimal numbers). While the format shows 12 hex characters, hyphens or colons to show grouping separate each pair. Each pair of characters represents a single hexadecimal number. The delimiters (i.e. non alpha-numeric ASCII characters--including spaces) show that the characters between them are to be taken as a single quantity. In decimal individual numbers can be written 1 2 3 or 1, 2, 3 etc.; however, 123 is one number or quantity--one hundred twenty three. In much the same way E 4 B or E:4:B represents three individual hex numbers (i.e. 14, 4, 11); however, E4B represents 3,659 (in decimal). Am I wrong?

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swmorris Wed, 05/02/2007 - 18:11
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A hexadecimal number gives you one of 16 possible values (the definition of hexa-decimal). 16 values would only be 4 bits of information (a nibble).

Therefore it takes two hex characters to equal a byte.

A MAC is 48 bits long, which as you noted is 6 bytes. However, to express that in hex, that would be twice as many characters (12) in order to do that.

The method of display is really just for our (human) benefit. The router doesn't care.

Some IOS versions will show it as four hex characters at a time with periods (e.g. 1122.3344.5566) whereas others will use hypens (e.g. 11-22-33-44-55-66) between the bytes. Most unix systems use colons (e.g. 11:22:33:44:55:66), but in the end they are all the same representation.

Also, what you are noting as the single character with colons is really more a unix thing. Leading 0's can be discarded. IOS will do this with IPv6 addresses, but not with MACs (at least as far as I have ever paid attention).

But the deal there is we (humans) are supposed to KNOW there are two hex characters between each colon, therefore if there's only one, assume there's a 0.

So a MAC of 0010.0123.4005 could also be seen as 0:10:1:23:40:5 and technically still be accurate.

Kinda irritating isn't it?!?!?



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gr84sight Thu, 05/03/2007 - 07:33
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Thank you for confirming what I was thinking. I do understand about hex digits requiring a nibble (that's how I convert) and about the leading 0's. My point is that there are errors in the training material. I'm deeply concered about this issue. That means that it is highly likely that there are errors on the official tests as well. I know that there were errors on the CompTIA exam when I got certified. I've made multiple attempts to contact Christine Yoshida, Cisco's CCNA Prep Center officiator, but she hasn't returned any of my e-mails or call. This was only one example. I've literally got doezens! Frankly, I'm surprised no one has brought suit against CompTIA or Cisco! When an organization grants certifications based on faulty tests (i.e. answers that clearly disagree with answers in training materials or are patently wrong) they need to be brought to accountability! Please understand that I am not angry with you. You're the first person to even reply to one of my inquiries. Cisco Training Partners are putting out erroneous study materials and those who use them are being improperly trained. The least experienced amoung them may fail their certification because of it! I'm simply proposing that Cisco needs to have an open channel for those seeking certification to get answers and report errors in training material so that future releases of said training material can be corrected! Would you be able to put me in touch with the appropriate party to address this issue!?


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