Returning to the era of 'duplicated' Ethernet addresses

July 3, 2010

Once upon a time back in the old days, Sun caused quite a stir by deciding that they would make the Ethernet addresses for their hardware be an attribute of the machine, not of the network interface. Or to translate, Sun machines used the same Ethernet address on all of their interfaces instead of doing what everyone else did, which was to have a different Ethernet address on each interface. This is spec-legal but caused various sorts of annoyances for system administrators and network designers; among the set of people who care about this stuff at all, there were many who felt that Sun had made the wrong decision and life was just simpler when multi-homed machines had different MAC addresses on different interfaces.

Over time this became less important as Sun's servers became less popular, and today this particular bit of trivia is probably long forgotten in most places. Even Sun's x86 servers used per-interface Ethernet addresses (and for all I know, Sun gave up on this in the end on their SPARC machines too).

Well, guess what. Those days are back, courtesy of VLAN-aware hosts. Such a machine has only a single physical network interface with a single Ethernet address, but that single interface is used by multiple networks through VLAN tagging. Outgoing Ethernet frames on each VLAN generally have no choice but to use the physical interface's Ethernet address, and so the host will be seen as having the same Ethernet address on different VLAN networks.

(There might be a use for this in detecting whether a host is using VLANs or physically separate network interfaces, assuming that you even care.)

In short: Ethernet addresses are now back to being only unique on their particular network, not globally unique across your entire set of networks. Hopefully no switches will explode.

(However, you're less likely to run into problems with this than people were in the Sun era. Back then, problems ensued because there were legitimate situations where people wanted to connect two network interfaces from the same machine onto the same network and tell them apart.)

Written on 03 July 2010.
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Last modified: Sat Jul 3 01:35:04 2010
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