2 Identical MAC addresses on the same network

ccb056 · Post by **ccb056** » Mon Sep 12, 2005 10:01 am

What happens if:

I clone the MAC address of a PC to another PC or router, both are on the same network, both have the same MAC address at the same time.

?

fliptw · Post by **fliptw** » Mon Sep 12, 2005 10:17 am

the exact same thing when half the cars on the race track suddenly lose steering control.

ccb056 · Post by **ccb056** » Mon Sep 12, 2005 12:25 pm

The MAC addresses travel the path of least resistance? (in a perfect world, a straight line)

question, are these cars in motion when they loose steering control, because if they are in the pits, nothing would really happen

Top Wop · Post by **Top Wop** » Mon Sep 12, 2005 12:37 pm

That would be impossible since every MAC address in the world is made to be unique. But by the freak chance that it is true then chances are they would continue to work but bandwith would be really slow for those 2 machines. If the router is hooked up to those 2 machines then the router would detect that there are clone mac addresses and may do something about it, like shutting down one or the other or both. In which case its a waste of time since you should just throw out the damn NIC and get a cheap one at Office Max for ten bucks.

Jeff250 · Post by **Jeff250** » Mon Sep 12, 2005 2:50 pm

I had a cloned mac address on my router like that at my folks' place. Never really thought about that implication, but I never ran into any problems.

DCrazy · Post by **DCrazy** » Mon Sep 12, 2005 2:59 pm

The cloned MAC only applies to the WAN side of the router. The LAN side still sees the router's original MAC.

pATCheS · Post by **pATCheS** » Mon Sep 12, 2005 3:20 pm

If two machines shared the same MAC address on the same network, packets coming from other devices would be received by both devices. It would be okay as long as only one of the two machines acts on any directed incoming data, but this is not guaranteed anymore since now both machines are receiving the same packets. Now imagine if the machines with identical MACs were both web servers

It'd cause all kinds of errors.

Capm · Post by **Capm** » Sun Sep 18, 2005 10:12 pm

Well... If they had different static IP's it may work for some apps, since I believe stuff is routed to the IP address and not the mac address first. And I say static because the same Mac address on DHCP would likely try to pull the same IP

DCrazy · Post by **DCrazy** » Sun Sep 18, 2005 10:20 pm

IP addresses are converted to MAC addresses via ARP (Address Resolution Protocol), which operates on a lower level of the OSI model. The problem would arise in the Ethernet layer, not the IP layer.

IPX/SPX and AppleTalk-over-Ethernet would have the same problem, and there's no IP (much less TCP/IP) involved there anywhere.