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On one computer, I am able to do a unique test, because the onboard NIC still partially working but is unstable (because of the lightning strike). It gets DHCP but then loses the connection for a bit and then reconnects. I installed a new NIC and the connection is rock solid (I tested this by connecting it directly to the modem: see point 6. above), but when connected to the internal network, I get no DHCP from the server as in the original symptoms described above.

On one computer, I am able to do a unique test, because the onboard NIC is still partially working but is unstable (because of the lightning strike). It gets DHCP but then loses the connection for a bit and then reconnects. I installed a new NIC and the connection is rock solid (I tested this by connecting it directly to the modem: see point 6. above), but when connected to the internal network, I get no DHCP from the server as in the original symptoms described above.

Update

I have new symptoms.

The problem is "spreading".

Recently, a crazy lightning storm that came through a modem knocked out several pieces of equipment (it actually knocked out a bunch of equipment around town, according to my contacts). It seems the strike propagated through the modem, out its ethernet ports, and then knocked out several network devices. It took out an ethernet port on my gateway, completely killed a VoIP box, went back out through that box's ethernet, knocked out several switches, and from there also damaged some of the ethernet ports on my workstations, as well as some printers.

Now, I shouldn't get too off track with the storm, because it doesn't have anything to do with the problem that this question is about, which predates the storm, but the necessary repairs did reveal new symptoms.

After replacing all the damaged switches, I had everything mostly back up and running. But the workstations with new network cards are not getting DHCP from the Windows Server.

On one computer, I am able to do a unique test, because the onboard NIC still partially working but is unstable (because of the lightning strike). It gets DHCP but then loses the connection for a bit and then reconnects. I installed a new NIC and the connection is rock solid (I tested this by connecting it directly to the modem: see point 6. above), but when connected to the internal network, I get no DHCP from the server as in the original symptoms described above.

So, the computer gets DHCP when using the original (now damaged and flaky) NIC, but not when using the new NIC.

Is there something with Windows Server that would make it refuse to hand out an IP if it doesn't like the NIC for some reason?

Background

I have a domain-joined Windows 10 Pro machine that has been working fine on an internal Windows Server 2012 R2 domain (for this question, let's call it MYDOMAIN.COM) for years without any problems. The Windows server handles both domain tasks (Active Directory) and DHCP.

Both the computer and the server and the network have received no major changes - hardware, software, or configuration - in years as well, other than the always plausibly-culpable automatic Windows updates.

Problem

Recently it simply stopped registering correctly with the internal network and domain.

It does not receive a DHCP assignment from the server.

It does not recognize the network as MYDOMAIN.COM.

Diagnostics

1. All other computers (dozens) have no problems on the same physical network.
2. Have switched network cables and run long network cables directly to the server room to bypass any intermediate physical issues: same problem.
3. Have switched NICs: same problem.
4. Have installed a WiFi card and tried to connect to the network via WiFi: same problem.
5. Have tried changing the switch port and changing the switch entirely: same problem.
6. If I connect the computer directly to the modem: it works (and identifies the network as "Network").
7. If I connect the computer to my phone using USB (Remote NDIS based Internet Sharing): it works, but strangely it recognizes the connection as MYDOMAIN.COM (even though it is not, so why is it misidentifying this network that has a completely different subnet?)
8. If I switch out computers and try to connect a different computer using the same cable, network ports, and switch ports: it works.

Conclusion

Based on all the tests I've done so far, it seems something is wrong with the computer itself and not my network, and for some reason it just doesn't want to register with DHCP. It won't even register when using a different cable, a different NIC, or even a wireless NIC.

I have no idea what I should try next to troubleshoot this issue. Can someone give me some pointers?

Background

I have a domain-joined Windows 10 Pro machine that has been working fine on an internal Windows Server 2012 R2 domain (for this question, let's call it MYDOMAIN.COM) for years without any problems. The Windows server handles both domain tasks (Active Directory) and DHCP.

Both the computer and the server and the network have received no major changes - hardware, software, or configuration - in years as well, other than the always plausibly-culpable automatic Windows updates.

Problem

Recently it simply stopped registering correctly with the internal network and domain.

It does not receive a DHCP assignment from the server.

It does not recognize the network as MYDOMAIN.COM.

Diagnostics

1. All other computers (dozens) have no problems on the same physical network.
2. Have switched network cables and run long network cables directly to the server room to bypass any intermediate physical issues: same problem.
3. Have switched NICs: same problem.
4. Have installed a WiFi card and tried to connect to the network via WiFi: same problem.
5. Have tried changing the switch port and changing the switch entirely: same problem.
6. If I connect the computer directly to the modem: it works (and identifies the network as "Network").
7. If I connect the computer to my phone using USB (Remote NDIS based Internet Sharing): it works, but strangely it recognizes the connection as MYDOMAIN.COM (even though it is not, so why is it misidentifying this network that has a completely different subnet?)
8. If I switch out computers and try to connect a different computer using the same cable, network ports, and switch ports: it works.
9. If I manually configure the NIC using a static IP/gateway/DNS server: it works.

Conclusion

Based on all the tests I've done so far, it seems something is wrong with the computer itself at the software or configuration level, and not my network, and for some reason it just doesn't want to register with DHCP. It won't even register when using a different cable, a different NIC, or even a wireless NIC.

I have no idea what I should try next to troubleshoot this issue. Can someone give me some pointers?