bohemiancoast

You may also need to restart your router, as the problem may be there.

I've done this dance occasionally, and its usually solved by judicious power cycling and rebooting the macs concerned.

Do you have any access comtrols in place? Does the network pane in system preferences provide any useful diagnostics?

Router/base station is a (single) Airport extreme.

It doesn't have any DHCP settings becuase it tells me it's not connected to an Airport network. It would pick them up from the network, yes.

There are very few network settings because of this.

I have restarted every relevant bit of kit with a plug. Mostly several times.

Is the Airport network configured to use 802.11g only, and not operate in 802.11b compatibility mode?

Do the various TiBooks use 802.11g? Because we've had no problem with them. Provided that's right, this does all sound quite plausible; it is Airport rather than Airport Extreme.

TiBooks are 802.11b. AlBooks are 802.11g. iBooks can be either depending on age. You don't have any TiBooks to test with, do you -- I thought it was one AlBook and one iBook.

Specifically, now I'm at home and can check on my Airport Extreme, what is the setting for Mode in the bottom left corner of the AirPort tab in the AirPort Admin utility? Might be worth trying a different channel, too.

I found that clearing out any Airport-related Keychain entries helped me troubleshoot that sort of thing. And I agree with them ^.

Oh, yes, forgot the other weird thing, which is that the keychain utility doesn't open; it bounces around in the dock and then closes.

I suspect you're seeing an 802.11b/g interaction at work - though without further details I couldn't be sure.

One quick test would be to borrow a non-Apple wireless router off some one, and see what happens...

Computers are hard. Even if Macs are usually easier than most, it's only relative.

Since the old iMac can find the base station when it's running Airport Setup Utility, that means its Airport card and antenna are working. Since connecting via an ethernet cable works, that means the base station is at least mostly okay. This appears to isolate the problem to two possibilities: the network settings specific to Airport on the iMac, or access controls on the base station.

The suggestions about rebooting are good. If the base station is being used as a router with DHCP, clients will ask the base station to assign them an IP address when they come up. To ensure that clients get usable IP addresses, turn everything off, boot up the base station first, then one client at a time. After powering up each client workstation, go into its network/Airport settings and note its IP address. Try configuring the problem iMac exactly the same as the one that works, and power it up before the working iMac (to see if it might successfully take the working iMac's place).

Good luck!

I was assuming that you were running Airport Setup Utility when the iMac was not connected via ethernet.

Oh yes. Though that would have been the sort of mistake i might quite easily have made.

Thanks for the tips. I've rebooted endlessly, but will have another go. Usurping my new iMac's internet connection would be a disaster -- but it would at least identify the problem.

Generally speaking, Macs are vastly easier than most. This is unusually hard.