On Fri, 6 Apr 2001, Terry L. Ridder wrote: > hello; > > 'mac' addresses are assigned to the manufactuerer of > each wired/wireless ethernet card. each ethernet device > must have their own unique 'mac' address. therefore, > it would be extremely unwise to have a card with > made-up 'mac' addresses. Not really, you just need to make sure that no other cards on your network have the same address. This isn't terribly difficult to check, and a good network administrator should have a database of all MAC address to hardware to IP address mappings anyway. I have very occasionally encountered two cards from a manufacturer with the same address. This has happened only once or twice in my life, but it does happen, mostly because there are only so many bits for the manufacturer to play with, usually only 32 or 24 of the whole 48 bit address. This is why the manufacturer usually includes a DOS utility (for PC ethernet cards anyway) on their driver disk to allow changing the address in a permanent way. > about the only way to guarantee > uniqueness would be to collect several dead ethernet > cards or pcmcia cards soley for the purpose of knowing > what their 'mac' address is and using those. Or just pick a random one from the 'ethers.txt' list or some similar compendium of address range -> manufacturer assignments. Pick one from a manufacturer that you've never heard of, like Symbolics, and you have a very good chance of never having a collision with one of your other cards. In fact, the probability of having an address collision is fairly low in any case. IIRC there are also some explicitly unallocated ranges, similar to the IPv4 addresses like 10.0.0.0/8. You can set the mac address of an ethernet card under Linux both permanently and temporarily (eg, until next reboot). Donald Becker has tools somewhere to permanently modify a MAC address, and there are also tools available (on Freshmeat.net for instance) to *temporarily* change the MAC address of a card. You can put such a program into your init scripts to have it do this at every boot. A friend of mine has done this with a firewall for his home network. His ISP is very slow to update their DHCP static address mappings so he just worked around it by temporarily changing the MAC addresses on his hardware until they finally updated their DHCP servers. This worked beautifully. Donald Becker for one explicitly mentions with his utilities that *permanently* changing the MAC address of a card is probably not a good idea. And that it doesn't always work right, and if it doesn't then you can be left with nothing more than a worthless pile of silicon for your efforts. There's nothing really wrong with assigning your own MAC addresses, it's just a pain to keep track of. Very rarely will you encounter problems with doing so, like collisions between multiple cards. Your network should have sufficient separation between hosts anyway, such that address collisions never have a chance to tempt the probability of occuring. > it would > still be extremely difficult to convince any ethernet > card or pcmcia card to have more than one 'mac' address. There is AFAIK no way to assign multiple concurrent addresses to a device. Does anyone know otherwise? Is it even theoretically possible? I don't think so the way most cards are designed. Why would you want to anyway? 'james -- James A. Crippen <james@xxxxxxxxxxxx> ,-./-. Anchorage, Alaska, Lambda Unlimited: Recursion 'R' Us | |/ | USA, 61.2069 N, 149.766 W, Y = \f.(\x.f(xx)) (\x.f(xx)) | |\ | Earth, Sol System, Y(F) = F(Y(F)) \_,-_/ Milky Way.
