Q At least once a day my iMac 27” i7 shows a message that its Bonjour name is in use, and it then assigns itself another name with its number incremented. Why is it doing this?
A When your Mac starts up, it tries to establish itself on your network as a Bonjour device. You determine the name that it tries to assign itself, set in the Computer Name in your Sharing pane.
What is currently going wrong is that when it attempts this, it is being told that there is already a device on your network with that name, so to avoid a name clash it is using that name with a number suffixed to it. This is usually the result of a network misconfiguration.
If possible avoid using DHCP to obtain the Mac’s IPv4 address, preferring a fixed address on the same sub-net as your modem-router. Check that you do not have two DHCP servers running on your network, which can also cause odd problems like this. Sometimes resetting the PRAM, by restarting with the Command, Option, P and R keys held until you hear the startup chime again, clears the issue.
Updated from the original, which was first published in MacUser volume 29 issue 2, 2013.