Converting Ethernet and Token Ring MAC Addresses

2017-11-03 09:05:05

Problem

You want to convert the bit ordering of MAC addresses to see how they will look after passing through an Ethernet-to-Token Ring bridge.

Solution

The Perl script in Example 15-1 converts Ethernet addresses to the way they will appear when connected through a bridge to a Token Ring. It also performs the reverse translation of Token Ring addresses to Ethernet, which is identical.

Example 15-1. eth-tok-mac.pl

#!/usr/local/bin/perl # # eth-tok-mac.pl -- a script to convert Ethernet to Token Ring MAC # addresses when bridging with RSRB or DLSw # $convert[0] = "0"; $convert[1] = "8"; $convert[2] = "4"; $convert[3] = "C"; $convert[4] = "2"; $convert[5] = "A"; $convert[6] = "6"; $convert[7] = "E"; $convert[8] = "1"; $convert[9] = "9"; $convert[10] = "5"; $convert[11] = "D"; $convert[12] = "3"; $convert[13] = "B"; $convert[14] = "7"; $convert[15] = "F"; if($#ARGV != 0) {usage( );} $input_MAC = $ARGV[0]; # first split the incoming MAC into bytes $_ = $input_MAC; s/[.:-]//g; for ($i=0; $i*2 < length($_); $i++) { @input_bytes[$i] = substr($_, $i*2, 2); } for ($i=0; $i <= $#input_bytes; $i++) { $_ = @input_bytes[$i]; # first check that there aren't any illegal characters in this address if(/[^0-9a-fA-F]/) { usage( ); } if (length( ) == 2 ) { @output_bytes[$i] = $convert[hex(substr($_, 1, 1))] . $convert[hex(substr($_, 0, 1))]; } else { usage( ); } } print "the resulting MAC is: "; for ($i=0; $i < $#input_bytes; $i++) { print "@output_bytes[$i]-"; } print "@output_bytes[$#input_bytes] "; sub usage( ) { print "usage: eth-tok-mac.pl "; print " where is in the form HH:HH:HH:HH:HH:HH "; print " or HH-HH-HH-HH-HH-HH or HHHH.HHHH.HHHH print " (H is a hex number 0-F) "; print "The output is the converted MAC address. "; print "Note that this conversion is exactly the same whether converting "; print "from Ethernet to Token Ring or Token Ring to Ethernet. "; exit; }

The program is run as follows:

$ eth-tok-mac.pl 00-00-0c-f0-84-60 the resulting MAC is: 00-00-30-0f-21-06

Discussion

Token Ring uses a convention of most significant bit first when writing a byte. Ethernet, on the other hand, puts the least significant bit first. So when a bridge connects these two media, the MAC addresses of devices on the Ethernet side will look unfamiliar when viewed from the Token Ring side, and vice versa. The rule for converting from one to the other is relatively simple, however, because it just reflects this reversing of the bit ordering.

Table 15-3 shows how the conversion algorithm works.

Table 15-3. Converting Token Ring to Ethernet MAC addresses
Token Ring	Ethernet
Hex	Decimal	Binary	Binary	Decimal	Hex
0	0	0000	0000	0	0
1	1	0001	1000	8	8
2	2	0010	0100	4	4
3	3	0011	1100	12	C
4	4	0100	0010	2	2
5	5	0101	1010	10	A
6	6	0110	0110	6	6
7	7	0111	1110	14	E
8	8	1000	0001	1	1
9	9	1001	1001	9	9
A	10	1010	0101	5	5
B	11	1011	1101	13	D
C	12	1100	0011	3	3
D	13	1101	1011	11	B
E	14	1110	0111	7	7
F	15	1111	1111	15	F

This table converts each individual hex character in a MAC address, but there is more to it than this because a byte is eight bits long, not four bits like a hex character.

Suppose the Ethernet address is 0000.0cf0.8460. The third byte of this address is 0c. To figure out how this byte will look on the Token Ring side of the bridge, you first switch the two hex characters to get c0. Then convert each of these characters by using Table 15-1. The c becomes 3 and the 0 stays the same, giving a final value of 30. So converting the entire address similarly gives 00-00-30-0f-21-06.

The script provides an automated way to do these translations. If the 8 bits in a byte are numbered from 18, this algorithm simply flips the order so that they appear from 81. This is clearly identical to the inverse translation where the bit order is converted from 81 to 18. So both the script and the above manual technique work identically when converting Ethernet addresses to Token Ring or Token Ring addresses to Ethernet.

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