Notational conventions | FAQ | MAC Address Vendor Lookup
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Notational conventions

The standard (IEEE 802) format for printing MAC-48 addresses in a human-friendly form is six groups of two hexadecimal digits, separated by colons (:) or hyphens (-), in transmission order (e.g. 01-41-93-62-98-ab or 01:41:93:62:98:ab).

This form is also commonly used for EUI-64. Another convention used by networking equipment uses three groups of four hexadecimal digits separated by dots (.) (e.g. 0141.9367.98ab), again in transmission order.

The standard notation, also known as canonical format, for MAC addresses is written in transmission bit order with the least significant bit transmitted first, as seen in the output of the iproute2/ifconfig/ipconfig command, for example.

IEEE 802.3 (Ethernet) and IEEE 802.4 (Token Bus) both send the bytes (octets) over the wire, left-to-right, with the least significant bit in each byte first, whereas IEEE 802.5 (Token Ring) and IEEE 802.6 send the bytes over the wire with the most significant bit first. As a result, there may be confusion when an address in the latter scenario is represented with bits reversed from the canonical representation. For example, an address in canonical form 12-34-56-78-9A-BC would be transmitted over the wire as bits 01001000 00101100 01101010 00011110 01011001 00111101 in the standard transmission order (with the least significant bit first). But for Token Ring networks, it would be transmitted as bits 00010010 00110100 01010110 01111000 10011010 10111100, in most-significant-bit-first order. The latter might be incorrectly displayed as 48-2C-6A-1E-59-3D. This is referred to as bit-reversed order, non-canonical form, MSB format, IBM format, or Token Ring format, as explained in RFC 2469. The canonical form is generally preferred and used by all modern implementations.

When the first switches supporting both Token Ring and Ethernet came out, some did not distinguish between canonical form and non-canonical form and so did not reverse MAC address bits as required. This led to cases of duplicate MAC addresses in the field.

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