Brian and Nederfox:
Just to add my two bits on MAC addresses... you are
both correct. The key to the material you're quoting, Brian, is that the MAC address provides a unique physical address to each interface on a
network. Two networks attached together by a router at the physical layer can potentionally have a node in one segment that is identical to a MAC address in the other segment, and have no problems what so ever. However, each interface on an individual network
must be assigned a unique MAC address for it to work properly. If two interfaces have the same MAC address, "very strange things" will begin to happen with the devices using those interfaces, but only if they share the same physical layer. So, for example, my computer here at home, connected to a personal LAN which passes through a broadband router to my ISP, could potentially have the same MAC address as another machine connected to the same ISP network, since they are on separate physical networks (due to my broadband router).
To ensure that devices generally do not conflict with eachother at the Data Link Layer, manufacturers obtain an OUI (a unique 3-octet identifier that makes up the first portion of a registered MAC address) from the IEEE, the organization the ISO council has designated as the registration authority for OUI assignments. Combined with the OUI and a manufacturer assigned 3-octet identifier, a NIC can have an IEEE certified, globally unique MAC address. However, some (very few) manufacturers try to cut corners and save the cost of selling a standard's compliant interface. I know this first hand from a shipment of low-cost Ethernet NIC's a company I consult for received. All of the MAC addresses for the cards were factory set to zeros. It was a complete administrative nightmare until we figured out what was happening and why. Thankfully, in this case, Nederfox was correct in saying that many NICs have a user-configurable MAC address. By simply changing the MAC addresses on the individual cards, using a manufacturer firmware utility, to machine addresses that we knew would be unique to our physical network, the problem instantly went away. We could of contacted the IEEE and asked for our own OUI, but it wasn't necessary.
Generally speaking, most NIC's will let you change the MAC address through the drivers of the operating system. This will not permanently replace a unique MAC address if your NIC has been assigned one by the manufacturer, and it will return to it's factory set MAC address if the setting is cleared, or, in some cases, if the machine is rebooted. In Unix-like operating systems, this is accomplished through the ifconfig command. In Windows, most drivers have a "Network Address" setting in the Advanced tab of your NIC's device properties. Many Linksys broadband routers also possess the ability to clone a MAC address from a machine on the internal network to it's WAN port, in case your ISP only allows your original PC's MAC address to connect.
Now... what this all has to do with the original question is beyond me.
--
Adam
Chameleon Networks
adam@chameleonnetworks.net