Some users have been noting errors “Another device on the network is using your computer’s IP address” and our network monitoring has detected a number of flip-flops
involving Apple kit trying to use the same IP address. The culprit seems to be Apple’s ‘Internet Sharing’ software which causes a machine to use addresses which have not been assigned to it. The problem seems to be that it hi-jacks the IP-to-MAC translation, so that packets to the intended destination go to it instead.

It looks to be related to the sleep proxy code. There’s quite a long discussion about this. In particular:

“macs running snow leopard will act as bonjour sleep proxy servers when they have internet sharing activated. this is likely the reason you’re seeing sporadic occurrences on your network rather than the entire 10.6 installed base.”

Princeton’s approach to the problem is to prohibit the use of such features on their campus network:

http://www.net.princeton.edu/mac/internet-sharing-x

It is therefore important that you have this internet sharing feature disabled on your Apple kit and we may at some point configure our switches to block this kind of activity. There are other issues with this sharing feature (e.g. turning into a DHCP server) which would definitely impact on other users. In such a case, the offending kit would be required to be moved from the network until the issue was resolved.

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