My self-built PC dual-boots Windows 11 and Bazzite Linux, and Ethernet stopped working after about two months of trouble-free use. The Realtek PCIe 2.5GbE controller shows link lights and reports sending and receiving data at 1 Gbps, but neither operating system receives an IPv4 address from the router. Windows reports that no DHCP server was found, and the adapter sometimes assigns itself an unrelated autoconfiguration address with no default gateway. Other devices use 192.168.x.x addresses normally.
Wi-Fi works on this PC, and a housemate briefly confirmed that their laptop could access the internet through the same Ethernet cable and wall connection. My home server also works over Ethernet with a fixed address. I have power-cycled the router and PC, reset Windows networking, reinstalled and updated the Realtek drivers, disabled and re-enabled the LAN device in BIOS, tried different ports, and tested both automatic and manually assigned network settings.
When I manually configured an address, subnet mask, gateway, and DNS server, I could reach devices on my local network, including the home server, but still could not access the internet. Windows eventually identified the router, then reported that it could not reach the DNS server. The wired PC does not appear correctly in the router's connected-device list, and the router may not be seeing the adapter's MAC address. I would prefer not to reinstall either operating system. What should I test next to determine whether this is a faulty onboard adapter, a router/DHCP problem, or a firmware or configuration issue?
3 Answers
The quickest isolation test is to bypass the house wiring and router as much as possible: connect the PC directly to a known-good router port using a short cable, then test another device on that exact port and cable with Wi-Fi disabled. If another device gets DHCP but this PC does not, suspect the onboard Realtek controller, its firmware settings, or a bad MAC negotiation. If a USB Ethernet adapter works on the same cable, the onboard NIC is the likely failure. You can also try disabling energy-efficient Ethernet, green Ethernet, and similar power-saving options in the adapter properties, and update the motherboard BIOS if its release notes mention LAN fixes.
Boot a Linux live environment and test the Ethernet port there with NetworkManager set to automatic addressing. This separates an installed-OS configuration problem from a hardware or firmware problem. Check the interface details with commands such as `ip addr`, `ip route`, and `nmcli device show`; a working connection should have a normal private IPv4 address, a default route, and DNS servers. If both installed systems and a live environment fail in the same way, reinstalling either OS is unlikely to help.
The autoconfiguration address and missing gateway indicate that DHCP is failing; the IPv6 link-local address is normal and does not prove that IPv4 is working. First, return both IP and DNS settings to automatic. Then check the router's DHCP lease table and logs while disconnecting and reconnecting the cable. If the PC never appears there, the DHCP request may not be reaching the router or the router may be rejecting that adapter's MAC address. Also check whether MAC filtering, access control, or a DHCP reservation is enabled.
A static address can let you reach local devices without fixing the underlying problem. If the gateway is unreachable or the router has not accepted the adapter, changing DNS alone will not restore internet access.
Failure in the live environment still would not prove the motherboard is defective, since the router, wall wiring, and adapter firmware could all be involved. Testing with a known-good USB Ethernet adapter is a useful comparison.