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So I think I have eliminated the cable, router, on board adapter, NIC and any driver issues.

Now the last thing I've tried - wireless

Disabled all network adapters and installed a new usb wireless dongle with new drivers. It found the signal and the network, allowed me to log into the network with my password and shows good signal and link quality. However the dongle's ip address is set to 169.254.42.249 with a subnet mask of 255.255.0.0 and a gateway of 0.0.0.0. Hence I still cannot ping the router via the wireless card either and of course cannot reach the internet. Renewing my IP address just gets the same values again!

So I think I have eliminated the cable, router, on board adapter, NIC and any driver issues.

• Used a RJ45 cable tester on the cable to eliminate cable issues - all pairs are fine.
• Plugged my laptop into the same cable instead of my pc to eliminate the pc - laptop connects to the network and internet ok
• Changed the router for a different one to eliminate the router - laptop connects, PC still doesn't.
• Disabled the onboard network adapter and inserted a new PCI NIC with freshly installed drivers instead to eliminate adapter HW issues. Still no connection, same, non working, symptoms.
• Took out new PCI adapter, re-enabled on board one and restored my entire HDD from an image backup from several days ago before any issue started to eliminate software or driver issues problems. - exactly the same, non working symptoms
• Disabled the onboard adapter again and reinstalled the PCI NIC and drivers with re-imaged HDD. Same, non working, symptoms.
• Disabled the BitDefender AV - same issues.
• Checked router via my laptop, all looks in order except that my old PC name is missing from the dhcp client list (as I would expect).
• Changed to a static IP address and entered the default gateway manually in both adapters separately with the other one disabled. set up the router to accept that static address - no difference.
• Removed the Win 7 PC from the homegroup and made sure the profile is home/work.
• Removed the new win 10 machine from the network
• Numerous times I've reinstalled the drivers, ran ipconfig /flushdns, nbtstat -RR, netsh int ip reset, netsh winsock reset, ipconfig /release,ipconfig /renew, turned off router and pc for an hour and restarted etc.

Running wireshark and capturing the pci nic I can see incoming packets from other machines and can see the router scanning to see which device has which IP address and that device replying, although clearly it does not get a reply from the win 7 machine as its default ip address is 169.245.245.158 as its not getting one from the DHCP (using dhcp with no static IP set up)

• Used a RJ45 cable tester on the cable to eliminate cable issues - all pairs are fine.
• Plugged my laptop into the same cable instead of my pc to eliminate the pc - laptop connects to the network and internet ok
• Changed the router for a different one to eliminate the router - laptop connects, PC still doesn't.
• Disabled the onboard network adapter and inserted a new PCI NIC with freshly installed drivers instead to eliminate adapter HW issues. Still no connection, same, non working, symptoms.
• Took out new PCI adapter, re-enabled on board one and restored my entire HDD from an image backup from several days ago before any issue started to eliminate software or driver issues problems. - exactly the same, non working symptoms
• Disabled the onboard adapter again and reinstalled the PCI NIC and drivers with re-imaged HDD. Same, non working, symptoms.
• Disabled the BitDefender AV - same issues.
• Checked router via my laptop, all looks in order except that my old PC name is missing from the dhcp client list (as I would expect).
• Changed to a static IP address and entered the default gateway manually in both adapters separately with the other one disabled. set up the router to accept that static address - no difference. Changed it back to dhcp again as it made no difference.
• Removed the Win 7 PC from the homegroup and made sure the profile is home/work.
• Removed the new win 10 machine from the network
• Numerous times I've reinstalled the drivers, ran ipconfig /flushdns, nbtstat -RR, netsh int ip reset, netsh winsock reset, ipconfig /release,ipconfig /renew, turned off router and pc for an hour and restarted etc.

Running wireshark and capturing the pci nic I can see incoming packets from other machines and can see the router scanning to see which device has which IP address and that device replying, although clearly it does not get a reply from the win 7 machine as its default ip address is 169.254.245.158 as its not getting one from the DHCP (using dhcp with no static IP set up). At one point I can even see an outgoing request from the NIC (from the default address of 169.254.245.158) to address 169.254.255.255 with my computer name in it using the browser protocol. Although I don't know at which physical point wireshark intercepts the traffic.

in response to a comment The local router's lan IP is 192.168.1.1 (that's what I can reach from my laptop but not from the PC) DHCP IP poll start 192.168.1.2 DHCP IP pool end 192.168.1.254 Lease time 86400

More info - wireshark

Running wireshark and capturing the pci nic I can see incoming packets from other machines and can see the router scanning to see which device has which IP address and that device replying, although clearly it does not get a reply from the win 7 machine as its default ip address is 169.245.245.158 as its not getting one from the DHCP (using dhcp with no static IP set up)

in response to a comment The local router's lan IP is 192.168.1.1 (that's what I can reach from my laptop but not from the PC) DHCP IP poll start 192.168.1.2 DHCP IP pool end 192.168.1.254 Lease time 86400