Ethernet MAC

Ethernet hardware addresses are 48 bits, expressed as 12 hexadecimal digits (0-9, plus A-F, capitalized). They might be written unhyphenated (e.g., 123456789ABC), or with one hyphen (e.g., 123456-789ABC), but should be written hyphenated by octets (e.g., 12:34:56:78:9A:BC). This gives a theoretical 281,474,976,710,656 addresses. This is more than 56,000 MAC addresses for each person on the planet!

In practice, the address assignment policy (using flad addressing) will inevitably lead to some wastage - but even so, there are likely to be enougth addresses for ever computer, piece of networking equipment and a large numebr of household/office devices (heating systems, ovens, washing machines, fridges, drinks dispensors, security alarms, video recorders, ...) which could in the future be internet enabled!!!

Source Address Assignment

The 12 hex digits of source address consist of the first/left 6 digits (which should match the vendor of the Ethernet network interface) and the last/right 6 digits which specify the interface serial number for that interface controller vendor (this gives 256 cubed addresses - or 16.78 million separate serial numbers).

The special address of all zeros (as shown below) is reserved for nodes that currently have no configured MAC address:

00 00 00 00 00 00

The IEEE-assigned addresses allow each vendor to assign their own interface serial numbers (this is a flat addressing scheme), but also allows protocol monitors to examine the first 3 bytes of a frame address to determine the manufacturere of the interface card being used.

The addresses associated with interface cards are source addresses, not multicast nor broadcast, so the second hex digit (reading from the left) will be even, not odd. The following list identifies some of the blocks of assigned vendor MAC addresses (i.e. the first 3 bytes of a MAC source address). As an example, the Ethernet address:

08 00 20 00 70 DF

corresponds to an interface manufactured by Sun Microsystems (see below).

A more complete list follows:

00000C  Cisco
00000E  Fujitsu
00000F  NeXT
000010  Sytek
00001D  Cabletron
000020  DIAB (Data Intdustrier AB)
000022  Visual Technology
00002A  TRW
000032  GPT Limited (reassigned from GEC Computers Ltd)
00005A  S & Koch
00005E  IANA
000065  Network General
00006B  MIPS
000077  MIPS
00007A  Ardent
000089  Cayman Systems  Gatorbox
000093  Proteon
00009F  Ameristar Technology
0000A2  Wellfleet
0000A3  Network Application Technology
0000A6  Network General (internal assignment, not for products)
0000A7  NCD             X-terminals
0000A9  Network Systems
0000AA  Xerox           Xerox machines
0000B3  CIMLinc
0000B7  Dove            Fastnet
0000BC  Allen-Bradley
0000C0  Western Digital
0000C5  Farallon phone net card
0000C6  HP Intelligent Networks Operation (formerly Eon Systems)
0000C8  Altos
0000C9  Emulex          Terminal Servers
0000D7  Dartmouth College (NED Router)
0000D8  3Com? Novell?   PS/2
0000DD  Gould
0000DE  Unigraph
0000E2  Acer Counterpoint
0000EF  Alantec
0000FD  High Level Hardvare (Orion, UK)
000102  BBN             BBN internal usage (not registered)
0020AF  3COM ???
001700  Kabel
008064  Wyse Technology / Link Technologies
00802B  IMAC ???
00802D  Xylogics, Inc.  Annex terminal servers
00808C  Frontier Software Development
0080C2  IEEE 802.1 Committee
0080D3  Shiva
00AA00  Intel
00DD00  Ungermann-Bass
00DD01  Ungermann-Bass
020701  Racal InterLan
020406  BBN             BBN internal usage (not registered)
026086  Satelcom MegaPac (UK)
02608C  3Com            IBM PC; Imagen; Valid; Cisco
02CF1F  CMC             Masscomp; Silicon Graphics; Prime EXL
080002  3Com (Formerly Bridge)
080003  ACC (Advanced Computer Communications)
080005  Symbolics       Symbolics LISP machines
080008  BBN
080009  Hewlett-Packard
08000A  Nestar Systems
08000B  Unisys
080011  Tektronix, Inc.
080014  Excelan         BBN Butterfly, Masscomp, Silicon Graphics
080017  NSC
08001A  Data General
08001B  Data General
08001E  Apollo
080020  Sun             Sun machines
080022  NBI
080025  CDC
080026  Norsk Data (Nord)
080027  PCS Computer Systems GmbH
080028  TI              Explorer
08002B  DEC
08002E  Metaphor
08002F  Prime Computer  Prime 50-Series LHC300
080036  Intergraph      CAE stations
080037  Fujitsu-Xerox
080038  Bull
080039  Spider Systems
080041  DCA Digital Comm. Assoc.
080045  ???? (maybe Xylogics, but they claim not to know this number)
080046  Sony
080047  Sequent
080049  Univation
08004C  Encore
08004E  BICC
080056  Stanford University
080058  ???             DECsystem-20
08005A  IBM
080067  Comdesign
080068  Ridge
080069  Silicon Graphics
08006E  Concurrent      Masscomp
080075  DDE (Danish Data Elektronik A/S)
08007C  Vitalink        TransLAN III
080080  XIOS
080086  Imagen/QMS
080087  Xyplex          terminal servers
080089  Kinetics        AppleTalk-Ethernet interface
08008B  Pyramid
08008D  XyVision        XyVision machines
080090  Retix Inc       Bridges
484453  HDS ???
800010  AT&T
AA0000  DEC             obsolete
AA0001  DEC             obsolete
AA0002  DEC             obsolete
AA0003  DEC             Global physical address for some DEC machines
AA0004  DEC             Local logical address for systems running DECNET

Note that the first byte of the source address is always even (since the least significant bit, or first bit on the wire indicates that the address is a group address ).

Destination Address Assignment

The destination address is a valid source address, except for an address which has the lowest bit of the first byte set to '1'. These addresses, including the all 1's broadcast address FF:FF:FF:FF:FF:FF and the set of multicast addresses, are point-to-multipoint addresses and can never appear as the source address in an Ethernet frame. A frame must be sent by a single source.

Can you imagine the complications which would arise with learning bridges if this rule was not kept? (This has beern seen in practice with some low-cost manufacturers cloning the same address in all their equipment - with immediate implications when a customer purchases and installs two items of the equipment in the same network!)

Internet Multicast Destination Address Assignment

Multicast destination addresses also comprise an OUI and a unique number, but in the case of IPv4 multicast, the former part is always 0x01005 (an OUI assigned to the IETF, shown highlighted in orange) and a group identifier (shown highlighted in yellow) that is derived from the Layer 3 multicast address. This address is mapped to the layer 2 destination address when multicast IP addresses are sent on an Ethernet LAN.

Gorry Fairhurst - Date: 10/03/2009