CCNA Exam: MAC address

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
tjieAsked:
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Don JohnstonConnect With a Mentor InstructorCommented:
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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rbeckerditeConnect With a Mentor Commented:
It is using hexadecimal to represent numbers this should explain:
http://en.wikipedia.org/wiki/Hexadecimal
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that1guy15Connect With a Mentor Commented:
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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