Network problem with one particular iMac in a home network

Question

I want to report a problem involving one particular iMac (2008 Al 24").
On my home network (Apple Airport Extreme and Express extensions, about five Macs and couple of iDevices), all other Macs can see and connect to each other as well as printing to the networked Brother printer without problem.
However, on one particular iMac, I can see all other Macs, can set Brother printer as default with green dot next to it in Print Preference Pane, but I can not connect to other macs, nor print to the printer. Meanwhile, the iMac can get on the internet without any problem. I checked Share Preference Pane and all settings are same as other macs.
The iMac used to work flawlessly until recently, and I first noticed the problem because I could not print, getting a message the printer can not be found (and the dots turn yellow for a while then went back to normal green). Then, I found neither the iMac, nor other macs on the network can connect to each other, even though I can see all of them in Finder.
I opened Apple Remote Network on my own mac, and I can see and connect all other shared macs, but this iMac's status showed not online, even though the IP adress is valid. As a results, no other macs can connect to the iMac, either. It seems to be a problem with the intranet only, not internet.
One more thing, about a month ago, the iMac popped out a warning about network name, saying the old name was in use (conflict) in the network, so I have to change it in Share Pane. Not sure if this is related to the intranet problem.
Reboot and repaired permission several times, no change. Hate to reinstall everything. Any help would be greatly appreciated.

Answer 1

Another update:
For unknown reason, my iMac was suddenly back onto the home "intranet". It can be accessed by my MacBook Air through Finder Connect, and can also be seen by Apple Remote Desktop.
However, printing is still a problem while other macs have no problem printing to the same printer. Whenever I send a print task, either Word document, or other programs such as GraphicConverter image, the iMac tried to connect to the default printer, but failed, and wanted to try again in 30 seconds 9and continued to fail). Meanwhile, the green dot next to the printer selection turned to yellow in Preference Pane. But as soon as I cancel the print job, the printer status went back to normal green.
The printer is a Brother color laser HL-4070CDW, connected through wireless (g only), with it own DHCP but specific IP address. I printed out a setting for the printer and all looks normal, and all other macs can print to it without problem. Other macs can also get access to the printer profile, and check inks copy counts, etc, but the iMac cannot.
Only thing changed since last post was a new update for Apple Airport firmware (to 7.5.2 I think). So my problem is still half solved: can connect to other macs on the same network, but still not be able to print.