When Ethernet will not connect, work from the physical layer up: confirm link lights on both ends, swap the cable, try a different router port, then reset the network adapter and renew the IP lease. Most wired failures come down to a dead cable, a stuck DHCP lease, or a disabled adapter, not the ISP.
Symptoms
- No link light on the NIC or router port when the cable is plugged in
- Windows shows "Unidentified network" or "Network cable unplugged"
- macOS Network preferences lists the Ethernet interface as "Not Connected"
- IP address begins with 169.254.x.x, indicating DHCP failed
- Connection drops repeatedly or negotiates at 10 Mbps instead of 1 Gbps
Common Causes
Damaged or unseated patch cable
A bent RJ45 clip, crushed cable, or half-inserted plug breaks one or more pairs. Gigabit needs all four pairs intact, so partial damage often produces no link at all.
Failed switch or router port
Individual ports die from static, power surges, or wear. The rest of the switch keeps working, which hides the fault until you test another port.
Disabled or corrupted NIC driver
Windows updates occasionally roll back or disable Realtek and Intel adapters. On macOS, a stale network location can bind the interface to the wrong service order.
DHCP or static IP misconfiguration
A stuck lease, duplicate IP, or a leftover static address from another network prevents the client from reaching the gateway even when the link is up.
VPN or third-party firewall filter driver
Uninstalled VPN clients sometimes leave TAP adapters and WFP filters behind that block the physical Ethernet interface from passing traffic.
Step-by-Step Fix
- Verify link lights on both ends before touching software
Look at the LED next to the RJ45 jack on the PC and the matching port on the router or switch. A steady or blinking light on both sides means Layer 1 is fine and the problem is higher up. No light on either end means cable or port. - Swap the patch cable with a known-good Cat5e or Cat6
Do not just reseat the connector. Replace the entire cable with one you have tested on another device. Cheap cables fail at the crimp; look for a wobbly RJ45 boot or a missing locking tab as the giveaway. - Move to a different port on the router or switch
Plug into ports 2, 3, and 4 in turn. If port 1 is dead but the others work, the port is toast and you can simply avoid it. If every port fails with a working cable, the switch itself needs a power-cycle or replacement. - Power-cycle the modem, router, and PC in order
Shut down the PC, unplug the modem and router, wait 60 seconds so DHCP leases and ARP tables clear, then power the modem first, the router next, and boot the PC last. This resolves stuck leases and hung switch ASICs. - Reset the NIC in Device Manager or System Settings
On Windows, open Device Manager, expand Network adapters, right-click the Ethernet controller, choose Disable, wait five seconds, then Enable. If it stays greyed out, select Uninstall device and reboot so Windows reinstalls the driver. On macOS, remove the Ethernet service under System Settings, Network, then re-add it. - Release and renew the IP address
Open an elevated Command Prompt and run ipconfig /release, then ipconfig /renew, then ipconfig /flushdns. On macOS, go to System Settings, Network, Ethernet, Details, TCP/IP, and click Renew DHCP Lease. A 169.254 address after renew means no DHCP server is reachable. - Rule out VPN filter drivers and firewalls
Check Control Panel, Network Connections for ghost adapters like TAP-Windows or Nord Tap. Uninstall leftover VPN software through Programs and Features, then reboot. Temporarily disable third-party firewalls to confirm they are not blocking the interface. - Test the NIC against a second device or the router directly
Bypass the switch and plug straight into a LAN port on the router. If that works, the switch is the fault. If it still fails, boot a Linux live USB; a working connection there confirms a Windows driver or configuration issue rather than hardware.
Quick diagnosis by what you observe
| Observation | Likely cause | First action |
|---|---|---|
| No link light on either end | Dead cable or unseated plug | Swap cable, reseat both ends |
| Link light on router only | Failed NIC or disabled adapter | Enable adapter, reinstall driver |
| Link up, IP is 169.254.x.x | DHCP failure | Renew lease, check router DHCP scope |
| Link up, no internet, gateway pings | DNS misconfiguration | Set DNS to 1.1.1.1 or 8.8.8.8 |
| Speed negotiates at 100 Mbps | Damaged pair in cable | Replace with tested Cat5e or Cat6 |
| Drops every few minutes | Duplex mismatch or bad port | Force auto-negotiate, try new port |
Prevention
- Label both ends of every patch cable so you can retire suspect runs quickly
- Keep one tested Cat6 cable and a USB-to-Ethernet adapter in your kit as spares
- Pin working NIC driver versions in WSUS or Intune to block bad rollouts
- Reboot edge switches on a quarterly maintenance window to clear memory leaks
FAQ
Why does my Ethernet work on another computer but not mine?
The cable, port, and switch are fine, so the fault is on your machine. Start with the NIC driver: uninstall it in Device Manager and reboot to force a clean reinstall. If that fails, check for leftover VPN filter drivers and reset the TCP/IP stack with netsh int ip reset.
What does a 169.254.x.x address mean on my Ethernet adapter?
That is an APIPA address, assigned by Windows when no DHCP server responds. It means the link is up but the client cannot reach a DHCP server. Check that the router's DHCP service is enabled, that no static IP is set on the adapter, and that no cable break is silently blocking the DHCP exchange.
Should I use a static IP to bypass DHCP problems?
Only as a diagnostic step, not a permanent fix. Set an unused address in the router's subnet with the correct gateway and DNS. If that restores connectivity, the underlying DHCP issue on the router or a rogue DHCP server still needs to be resolved before you revert to automatic addressing.
Contact our IT support team for hands-on help diagnosing persistent wired network faults.