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CCNA Exam: MAC address

Posted on 2009-03-30
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Hi,

1)I am still confused to decide "the lowest or the highest MAC address of the switches in the network"
2) For Example: This is the sample in one of the prep test
- Switch 1's MAC address: 002A.3050.0B05
-Switch 2's MAC address: 002A.3050.0B07
-Switch 3's MAC address: 002A.3050.0B15
-Switch 4's MAC address: 002A.3050.0B0B
3) The lowest MAC address per prep test's answer is 002A.3050.0B05 (The MAC address of Switch 1)
4) My questions: i) If we compare the MAC address of Switch 1, 2 and 3, it clearly seen that the MAC address of Switch 1 is the Lowest MAC address, BUT what about if we compare the MAC address of Switch 1 and Switch 4? (How we make sure that the MAC address of SWITCH 1 is still the lowest one?) (Please answer it with your own wordings), ii) How is  the FORMULATION of the MAC address?; how to CALCULATE it (if there are COMBINATION between the NUMBER and the ALPHABET of the MAC address?) (please provide the answer as detail as possible)
5) Thank you

Tjie
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Question by:tjie
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by:rbeckerdite
rbeckerdite earned 100 total points
ID: 24022962
It is using hexadecimal to represent numbers this should explain:
http://en.wikipedia.org/wiki/Hexadecimal
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Don Johnston earned 250 total points
ID: 24023368
MAC addresses are expressed in hexadecimal (hex) notation. In hex, we notate each four bits (nibble) with a character. So now we can either refer to eight bits as a "byte" or we can call it two "nibbles".

With hex, each nibble is represented by a single character. Because four bits has 16 different values (0 - 15), values 10 - 15 are represented by the letters A -F. So in hex, you don't say a nibble has a value of "12", instead it has a value of "C".

So in summary:

0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
0110 = 6
0111 = 7
1000 = 8
1001 = 9
1010 = A (10)
1011 = B (11)
1100 = C (12)
1101 = D (13)
1110 = E (14)
1111 = F (15)

Now on to the question:

Since the first five bytes of the MAC address are the same for all the devices, we'll ignore those. This leaves the 6th byte for the devices as:

05
07
15
0B

Now the third one gets kicked out because the high order nibble is a "1" which means it's higher than the rest.

This leaves:
05
07
0B

Since the high-order nibbles are the same for these three devices, we'll focus on the low-order nibble.

5, 7 and "B".

Since most people think decimal by default, we'll convert...

H  D
5  5
7  7
B  15

Which one is lower? 5

15 (actually B) is the highest of the three.

Hope this helps.
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by:that1guy15
that1guy15 earned 150 total points
ID: 24023370
Make addresses are more commonly represented in 8 bit groups. So your example above (002A.3050.0B05) would be broken down as such 00-2A-30-50-0B-05.

Each group is then broken down further into 4 bit sections so the last group (05) would look like this in binary: 0000 0101.

Each 4 bits must be represented by a single character so once you get past 9 then you run into issues (10 -15 have two characters) so letters are used.

Hex to numeric conversion is as such:

1- 9 equals 1 - 9

10 = A
11 = B
12 = C
13 = D
14 = E
15 = F


So in your example above:

0B-05 = 0 11   0 5  or 0000 1011  0000 0101
0B-0B = 0 11   0 11 or 0000 1011 0000 1011

Since the last number (5) is lower than (11) the first answer is lower.

Hope that helps
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