Bit ordering in MAC addresses

Ethernet LANs use a different representation of MAC addresses from that used by Token Ring. The order of the bits in each byte of the address on Ethernet is the reverse of the order on Token Ring. The local and remote nodes are usually either on the same LAN or on LANs of the same type connected by a bridge; in both cases, the nodes will both use the same representation of the MAC address, and no conversion is required.

If the two nodes are on LANs of different types (one Ethernet, the other Token Ring) connected by a bridge, you will usually need to reverse the bit order of each byte of the address when specifying a remote MAC address. To reverse the bit order, take the following steps:

Reversing the Bit Order in a MAC Address

1. List the MAC address as six bytes, with each byte represented by two hexadecimal digits.
2. Swap the order of the two digits of each byte.
3. Convert each digit as shown in Table 1.

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Table 1. Bit Conversion for MAC Addresses

0->0	8->1
1->8	9->9
2->4	A->5
3->C	B->D
4->2	C->3
5->A	D->B
6->6	E->7
7->E	F->F

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Table 2 illustrates steps 1, 2, and 3:

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Table 2. MAC Address Bit Conversion Example

List the MAC address	1A 2B 3C 4D 5E 6F
Swap the digit order	A1 B2 C3 D4 E5 F6
Convert each digit	58 D4 3C B2 7A F6 (the bit-reversed form of the original address)

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