What do I do when I continually get the message 'another device on the network is using your computer's IP address' on my iMac?

Question

Hwe continually get this message on our iMac even when other devices in the same room can access the network (laptop, iPad etc). We have tried using a fixed IP address and also an automatic IP selection but to no avail. Any suggestions,

Answer 1

What that means is you were forcing your Mac to always use the network address 123.456.7.89 since you had manually assigned that. Normally this would be fine, your router would check for active addresses in use and assign a different address to any computer or device trying to join the network. The problem could arise if your Mac is off-line and the router has already assigned the IP address to one of your other computers. When you tried to log in with your Mac you would get the conflict message that you have noticed.
Setting all computers to using DHCP will let the router handle the addressing.
I would check all of your computers and PC's. You may have more than one of your computers set to manually assign an address using that same IP address. Make sure you set all of them to DHCP only. Also, check if you have any other IP devices like a printer or NAS drive that is using static addressing. Those can cause the same issues.
It is prefered to use static addressing with those as it makes it easier to find them on the network and since they are pretty much always left on, the router knows they are there.
On your Mac go to System Preferences > Network > Advanced > TCP/IP:
What is shown should read:
Confugure IPv4
IPv4 Address
Subnet Mask
Router
Configure iPv4 using DHCP with manual address
Change that to "Using DHCP"
Apply the change and close the window.
I warn you, I've not yet received my first Mac and it's only an educated guess. I hope it helps.