Q: Trying to help a friend diagnose his wi-fi problem

Consider trying a public DNS server. I don't know about Japan but a couple in the U.S. are Google and OpenDNS

Google Primary DNS 8.8.8.8

Google Secondary DNS 8.8.4.4

OpenDNS Primary 208.67.222.222

OpenDNS Secondary 208.67.220.220

To try a different DNS server, in the Network preference pane, click the Advanced button (lower right).

Click the DNS tab. If there is anything in the DNS Servers box, click each line and then the minus (–) button to delete them.

Then, click the plus ( + ) sign and enter a public DNS address. It's recommended to enter a primary and secondary DNS address.

What I have not seen in this thread is whether you had your friend power cycle all the equipment. That is turn off computer(s), router, and modem. Wait at least 2 minutes and then power the equipment back on in reverse order but be sure to allow each device to complete its startup routine before turning on the next device.

But this friend's iMac's Internet connection has just gone wonky.

...

I suppose it's possible his wi-fi hardware has gone bad. Does that ever happen?

Oh yes. What you describe about the iMac was my experience earlier this year and the culprit was the ISP's modem. See this thread.  Note: it could be the router. To determine which, i.e., modem or router, remove the router from the network and connect one of the computers directly to the modem. If the Internet connection is fine then it might be the router. If the Internet connection is wonky then it just might be the modem.

Have you tried sharing the connection from the iMac and trying to join that with the acc that can't join the network currently

Progress...Terrific! Thanks for the star.

I actually meant the wi-fi in his iMac, not the ISP modem or wi-fi router. I think both of those are ok because other devices are connecting just fine with them. Also, above I did mention that we recycled everything: his iMac, the modem and the wi-fi router.

Anything and everything is subject to glitches and failure. In this case probably not since the iMac also became wonky. In one of your posts yesterdays you stated

He tried restarting the Mac twice already. And we did try to power off and power on the router.

which did not include the modem. Every device in the network is a possible culprit.

As for the connection error with iTunes, I get that frequently. With all the changes you had him try, including the latest change to Google DNS, his computer (and possibly the iMac) may be confused or a bit slow to catch up with all the changes. Try rebooting the computers.

Another suggestion is to change the wi-fi channel in the router's settings. An ISP tech recently told me that wifi channels can get "stale" so changing the channel can help.

As for this being a recent problem, it's possible that the router settings file is corrupt. If he has a backup of his router settings, try reloading the backup settings file. Or another file on his computer has become corrupt that affects some other files. Since he uses OS 10.6.8, he could run Disk Utility from the original Install CD and verify/repair the hard disk.

HTH

It should appear as a wifi network but it seems its just another on the list of things that are not working.

Could anything else see it, (an iPhone maybe)

It is, but the fact that you can't get it to work suggests another problem for you to sort out.

You share it with wifi, not bluetooth: when it's done it should look like this

In practice it connects via wifi or ethernet, clearly you have not yet tried it out. I suggest you do, it will at least tell us if the reluctant Mac can connect.

Did you try anything than the problem Mac, yes or no will suffice