Ethernet - Router not assigning IP addresses to some devices.

Understood. 

I was able to get the Windows computer to connect once, but now it will not. It says "Ethernet does not have proper IP address" - the iMac never has been able to connect and says "Self Assigned IP address - cannot connect to the internet" 

I've tried to avoid asking questions the other support techs should have already asked like, have you run the network troubleshooter in windows? What are the ethernet IP settings, and if they aren't dynamic (get addresses automatically) what are they? The IP address for everything on the local network need to be different, are they? I'll answer questions as best I can, but as you're finding out the problem can be hard to find.

I really appreciate your help! 

I have ran the troubleshooting in windows and connected to a live tech for my Mac through Apple (they advised this is a router issue). 

IP settings are the same for each computer, using DHCP to auto populate the addresses. 

@jenniferrr   Which AT&T Gateway do you have?  If it's the BGW320, try doing a factory reset (hold down the Reset button on the gateway for >15 secs).  We've just recently seen a user with the 320 have bizarre wired LAN connection issues and the reset fixed that.

Be aware that the reset will wipe out all of the configuration changes you may have made to the gateway so make a note of anything you might want to re-enter afterwards.

@tonydi Thank you for the reponse! I do have a BGW320 - just tried the reset and the issue is still occuring.

Everything else seems to be working fine. 

Just to add, both computers connect wirelessly - it's just the ethernet on both that stopped working Friday morning. 

What happens if you connect one of the problem computers directly to the BGW320, taking the switch out of the equation for that system?

I don't know what switch AT&T supplies.  Have you tried plugging it into a different port on the BGW, especially if you have it connected to the blue 5Gb port?

When I plug my iMac directly into the wall jack it has the same issue. Unfortunately it is too large for me to transport to a different area in the house to try other wall jacks. 

The Windows machine will intermittently connect via the wall jack in question, sometimes it will, sometimes throws the IP address error, but connects to via Ethernet via other wall jacks in the home.

For information sake, the tech set up three switches in my home, one in the living room, the office and rec room, so they are not directly connected to the BGW. 

Everything besides these two computers works great, ha!

I think we're getting crossed up with the nomenclature.  An Ethernet "switch" is a multiple port device much like what you see on the back of the BGW.  It's essentially like a power strip where you have one input that feeds to multiple outlets, except in this case the switch sends Ethernet signals instead of electricity.  A switch needs to be connected to the gateway in order to give you a network and internet connectivity.

Can you post some pictures, like the back of the BGW so we can see what's connected, and then where those wires go before they leave the area and head out into the rest of the house to those wall ports?

@tonydi, yes, the switch is exactly that. The way this house is wired is that everything runs to a switch which is then fed into the main cable to the router.

In the other rooms, there are items connected to a switch that feeds back into the hub that connects to the main cable connected to the BGW. At this time there is only one cable connected to the router. 

I originally thought the house was wired so each wall jack would feed into the router it's self, but it all goes to a "hub". 

But you're talking about multiple switches in one instance, but a single "hub" (switch?)

connected to the BGW in another sentence.

Pictures would really help me out here.