----- Original Message -----
From: brian
burke
Sent: Monday, December 04, 2000 12:07 AM Mike,
I recently had this same issue come up.
Luckily, I had a friend who seems to know everything about everything that
showed me the best way to understand it. I caught on in about 10
minutes. Let me illustrate it below.
1. Ethernet and Token Ring both read bytes from
left to right.
2. The difference is that they read the bits within
those bytes the exact
opposite. Ethernet starts with
bit 0 on the far right side of
the byte (least significant bit;
canonical) and reads to the left. Token Ring
does the exact opposite and
reads the bits from left to
right (most significant;
non-canonical).
Because of this the hardware must be able to make
the translation between the two. We start with
the numbers 0-9 and A-F for Hex. This is the way that MAC addresses are
expressed by both protocols just backwards (sorta..) Since we're
really translating binary here we see that each character in the MAC
address is represented by four bits (0000). Remember that we are going by
decimal 2s so the 4 bits would be (8421) represented in 1s and
0s. By taking the two outside bits and
trading their places and then taking the inside bits and doing the same thing we
make the conversion. Look below:
0 = 0
0000 0000
1 = 8
0001 1000
2
= 4 0010 0100
3 = C
0011 1100
4 = 2
0100 0010
5 = A
0101 1010
6 = 6
0110 0110
7 = E
0111 1110
8 = 1
1000 0001
9 = 9
1001 1001
A = 5
1010 0101
B = D 1011
1101
C = 3
1100 0011
D = B
1101 1011
E = 7
1110 0111
F = F
1111 1111
There is one more thing to remember. The MAC
that is being converted must be broken down into what is called niblets.
That is, in segments of two. For example, take the MAC address
00.10.a4.a9.01.3e. Notice that the dots show what I mean by segmenting it
up. Here comes the easy part. For each niblet switch the two
character's places. Then do the conversion from the chart above. Look
below:
00.10.a4.a9.01.3e Ethernet
00.01.4a.9a.10.e3
00.08.25.95.80.7c Token
I hope that this is helps and didn't confuse
you. If you have any questions about it please send me a
message.
Brgds,
Brian Burke
|