Ethernet Port Not Working — No Wired Internet Connection

Common Symptoms

• Laptop or desktop shows 'Network cable unplugged' even with cable connected
• No link light on switch port or device NIC when cable is plugged in
• Ethernet works on one device but not another on the same cable
• Wall jack Ethernet has no link light but direct connection to router works
• Connection shows as connected but no data passes (1 Gbps link, 0 throughput)
• Ethernet port worked yesterday but stopped after moving furniture or rerouting cable
• Switch port link light on but getting 10/100 Mbps instead of expected 1 Gbps
• PoE device (camera, access point) not powering up from switch port

Most Likely Causes

1. 1

   Cable Failure — RJ45 Crimp Failure, Kink, or Cable Swap Test

   The most common Ethernet failure cause is the cable itself, specifically the RJ45 crimped connector at either end. The RJ45 crimp attaches 8 copper wires to 8 gold contacts inside the connector — if any wire is not fully seated, or if the connector is damaged by being stepped on, rolled over by a chair, or bent at a sharp angle, continuity fails on one or more pairs. Even a single broken wire (open circuit on one pair) causes gigabit Ethernet to fail or fall back to 10/100 Mbps. Cable swap test: replace the suspect cable with a known-good cable you can test — even a 6-foot patch cable from a bag in the closet. If the new cable works, the old cable is failed. RJ45 visual inspection: look at the connector head-on with the tab facing down — you should see 8 gold contacts at the top, all at the same depth. If any contact appears lower or recessed, the wire is not making contact. Cable kink diagnosis: run your hand along the entire cable length feeling for sharp bends, kinks, or spots where the cable was stapled through the outer jacket. A cable damaged by a staple gun (common when running cable through walls) may have intermittent continuity that passes a quick test but fails under movement. Multimeter continuity test: set multimeter to continuity mode (beep). Touch the probe to pin 1 at one end and pin 1 at the other end — should beep (continuity). Repeat for pins 2 through 8. A proper cable tester (RJ45 cable tester, $12–$20) tests all 8 pairs simultaneously and shows wiring order.

2. 2

   Wall Jack Failure — Punch-Down Connection Loose or Oxidized

   Wall Ethernet jacks (keystone jacks) connect to the internal cable via a 110-type punch-down termination — 8 wires are pressed into IDC (insulation displacement contact) slots that cut through the wire insulation to make electrical contact. Over time, these connections can loosen from vibration, oxidize from humidity (especially in bathrooms or laundry rooms), or be pulled apart when furniture catches the wall cable. Diagnosis: if a cable plugged directly into the router or switch works, but the same cable plugged into the wall jack does not, the problem is in the wall jack, the in-wall cable run, or the patch panel at the other end. Wall jack visual inspection: use a flat-head screwdriver to pop the faceplate off. Inspect the punch-down block — wires should be fully seated in the IDC slots and color-coded correctly (T568B standard: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown for pins 1–8). Any wire that has pulled out or shows green oxidation should be re-punched. A punch-down tool (110 type, $10–$20) seats the wires and cuts the excess cleanly — a screwdriver push-down will make contact but is less reliable. If re-punching the jack doesn't help, test the in-wall cable run itself with a cable tester: disconnect both ends and test each pin for continuity. A failed in-wall cable (often damaged by staples during installation or by pests) requires either a new cable run or a replacement with surface-mounted conduit.

3. 3

   NIC Driver Issue or Disabled Adapter — Windows and Mac

   If the cable and switch port are verified good (link light is present on the switch when the cable is plugged in) but the computer still shows 'No Internet' or 'Network cable unplugged,' the problem is in the computer's network interface card (NIC) driver or settings. Windows diagnosis: (1) Press Win + X > Device Manager. Expand 'Network Adapters.' Look for the Ethernet adapter (e.g., 'Intel Ethernet Connection I219-V,' 'Realtek PCIe GbE Family Controller'). A yellow exclamation mark = driver error. Right-click > Update Driver > Search automatically. (2) If the adapter isn't listed at all or shows a red X, the NIC is disabled. Right-click > Enable Device. (3) Network reset: open Command Prompt as Administrator, run: 'netsh int ip reset' then 'netsh winsock reset' then reboot. (4) If a Windows Update recently installed a new NIC driver and Ethernet broke, roll back the driver: Device Manager > right-click adapter > Properties > Driver tab > Roll Back Driver. Mac diagnosis: (1) System Settings > Network. Look for the Ethernet adapter in the left panel — if it shows a red dot, the NIC isn't detecting a link. Try a different cable first. (2) Create a new Network Location (System Settings > Network > Location dropdown > Edit Locations > +) to reset network configuration. (3) Delete network preferences: navigate to /Library/Preferences/SystemConfiguration/ and delete NetworkInterfaces.plist and preferences.plist (requires admin password). Reboot and let Mac rebuild network config. A macOS update that breaks Ethernet is a known issue — check Apple Support Communities for your specific Mac model and macOS version.

4. 4

   Duplex Mismatch — Port Autonegotiates to Wrong Speed

   Duplex mismatch occurs when one side of an Ethernet link is set to full-duplex and the other is set to half-duplex — or when one side is hardcoded to 100 Mbps and the other expects 1 Gbps. Symptoms: the link LED is on (physical connection is made), but throughput is extremely poor (often less than 1 Mbps), you see high error rates, or network performance degrades under load. The most common cause is a managed switch port hardcoded to 100 Mbps full-duplex connecting to a device NIC set to 'auto-negotiate' — the NIC autonegotiates to 100 Mbps but autonegotiates to half-duplex because the switch isn't participating in the autonegotiation handshake properly. Fix on Windows: Device Manager > Network Adapters > right-click NIC > Properties > Advanced tab > find 'Speed & Duplex' — change from 'Auto Negotiation' to the specific speed and duplex matching the switch port (e.g., '1.0 Gbps Full Duplex'). Fix on a managed switch: log into the switch admin interface and ensure the port is set to 'Auto' speed and duplex, not hardcoded to a specific value. For unmanaged switches, both sides must be set to Auto (the default for all modern NICs) — changing either side to a fixed value while the other is on Auto causes a mismatch.

5. 5

   PoE Overload — Switch Port Not Powering PoE Device or Port Shuts Down

   Power over Ethernet (PoE) delivers DC power to devices like IP cameras, wireless access points, VoIP phones, and smart home controllers over the same Ethernet cable that carries data. PoE problems are a subset of Ethernet port failures — the link may not come up at all if the PoE device can't receive its required power. PoE power classes: PoE (802.3af): up to 15.4W per port. PoE+ (802.3at): up to 30W per port. PoE++ (802.3bt): up to 60W or 100W per port. Overload scenario: a PoE++ access point (requires 30W) connected to a PoE (15.4W max) switch port will not power on — the switch's PoE controller detects the device needs more power than it can supply and refuses to energize the port. The switch port link light may flash or blink rapidly and never stabilize. Fix: verify the PoE standard required by the device (see its manual or label) and the PoE budget of the switch port. Upgrade to a PoE+ or PoE++ capable switch if needed. Also check total PoE budget: a 4-port PoE switch with a 60W total budget cannot power four 15.4W devices simultaneously — the last device connected may get no power. Safety note: PoE delivers 44–57V DC on the cable. This voltage is not dangerous to healthy adults in normal use but can damage a non-PoE device's NIC if a PoE injector or switch port is connected to a device that doesn't support PoE — always verify PoE compatibility before connecting.

Quick DIY Checks

1. 1Cable swap test: replace the suspect cable with a known-good cable. If the problem resolves, the original cable is failed — look for kinks, sharp bends, or a damaged RJ45 connector. A proper RJ45 cable tester ($12–$20) tests all 8 wires in 5 seconds.
2. 2Link light check: a link light (usually green or amber LED on the switch port and NIC) indicates physical layer connectivity. No link light = the physical connection is not established (cable, port, or NIC hardware issue). Link light present but no data = driver, DHCP, or duplex issue.
3. 3Isolate the segment: plug the cable directly into the router (bypassing switch and wall jack). If it works, the problem is in the switch, patch panel, or wall jack. If it still doesn't work, the problem is in the cable or NIC.

1. 4Windows NIC check: Win + X > Device Manager > Network Adapters. Look for yellow exclamation marks or disabled adapters. Try Update Driver or Enable Device. Run 'netsh int ip reset' and 'netsh winsock reset' in an elevated command prompt if the NIC appears healthy but still no connectivity.
2. 5Multimeter cable continuity test: set to continuity mode. Touch pin 1 on one end to pin 1 on the other end — should beep. Repeat for pins 2–8. Any pin without continuity identifies the broken wire pair. For T568B wiring order: 1=white-orange, 2=orange, 3=white-green, 4=blue, 5=white-blue, 6=green, 7=white-brown, 8=brown.

Repair vs Replace

Recommended Tools & Parts

• RJ45 Cable Tester (8-Port, Tests All Wiring Configurations)

  Essential diagnostic tool for any Ethernet troubleshooting. Tests all 8 wire pairs for continuity, shorts, and correct wiring order simultaneously. Includes remote unit for testing patch cables and wall-to-wall runs. Identifies crossed, reversed, or open wires instantly. Far more reliable than a multimeter for cable testing.

  $12–$25

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• Cat6 Ethernet Patch Cable — 6-Pack Assorted Lengths

  Cat6 patch cables for replacing failed patch cables at the switch, router, or device end. Cat6 supports 1 Gbps reliably and 10 Gbps up to 55 meters — future-proofs connections compared to Cat5e. Booted RJ45 connectors protect against snag damage. A 6-pack with assorted lengths (1, 3, 6, 10 ft) covers most home networking needs.

  $12–$20

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• 110-Type Punch-Down Tool with Cat5e/Cat6 Blade

  Required for re-terminating wall jacks and patch panels. Impact punch-down tool with reversible 110-blade for Cat5e/Cat6 terminations — seats the wire and cuts excess in one motion. Adjustable impact setting (low/high). Includes both cut and no-cut blade orientations. Essential if you have wall jacks or a patch panel.

  $10–$20

  Buy on Amazon →

• Keystone Jack — Cat6 (10-Pack, T568B)

  Cat6 keystone jacks for replacing failed wall jack terminations. T568B wiring standard (the standard for North American residential and commercial installations). Compatible with all standard single-gang and multi-gang wall plates. Color-coded IDC slots (A and B positions labeled). Replace the entire keystone module rather than re-punching a worn jack — keystone jacks cost $1–$2 each.

  $8–$20

  Buy on Amazon →

• 8-Port Gigabit Unmanaged Switch (TP-Link TL-SG108)

  TP-Link TL-SG108 is the most reliable and popular 8-port gigabit unmanaged switch for home and small office use. Plug-and-play (no configuration needed), auto MDI/MDIX (works with any cable orientation), and supports full wire-speed switching on all 8 ports simultaneously. Replace a failed switch or expand your wired network.

  $20–$35

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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