Full Diagnostic Tree & Step-by-Step Overview
Look at the front status LEDs of your standalone Cable Modem, DSL Modem, or Fiber ONT (Optical Network Terminal). What is the state of the 'Online', 'Internet', 'PON', or 'Cable' light?
- The Online/PON light is OFF, flashing red, or blinking orange continuously
- The Online/PON light is solid green/white, but the Router's WAN/Internet light is orange/amber or unlit
- All modem and router lights look solid green/white, but devices report 'No Internet Access'
What type of internet delivery infrastructure supplies your home?
- Cable Internet (Coaxial Cable screwed into modem)
- Fiber Internet (Optical cable running into an ONT box)
- DSL / Copper Phone Line
DOCSIS Upstream/Downstream Channel Lock Failure
Solution:
Diagnostic Cause
A flashing Downstream (DS) or Upstream (US) light indicates the modem cannot establish a frequency lock with the ISP headend (CMTS). This is typically caused by loose coaxial splitters, water-logged drop cables, or localized neighborhood outages.
Step-by-Step Fix
1. Inspect the F-connector on the back of the modem and wall plate; hand-tighten until snug.
2. Remove any unneeded signal splitters between the wall jack and the modem.
3. Perform a 3-minute hard reset:
Unplug modem power cord.Wait 3 full minutes to clear ISP CMTS session locks.Reconnect power and allow 5 minutes for DS, US, and Online LEDs to turn solid.4. If the Online light continues to blink, log into modem diagnostics (192.168.100.1) and check Power Levels: Downstream power should be between -15dBmV and +15dBmV, SNR should be >33dB. If outside these parameters, contact your ISP to dispatch a line technician.
Loss of Signal (LOS) / Fiber Cable Micro-Fracture
Solution:
Diagnostic Cause
A red LOS (Loss of Signal) light on a Fiber ONT indicates zero optical light reaching the receiver. Fiber optic jumpers (thin yellow/white cables) break easily if bent past their minimum bend radius.
Step-by-Step Fix
1. Inspect the thin fiber patch cable connecting the wall outlet to the ONT.
2. Ensure the green or blue SC/APC connector is fully seated into the ONT optical port with a firm click.
3. Ensure the cable is not pinched under furniture or bent sharply.
4. Critical: If the LOS light remains solid red, do NOT clean the glass tip yourself. Contact your Fiber ISP immediately; an optical break requires a field technician with a fusion splicer.
High Line Attenuation or Missing DSL Filter
Solution:
Step-by-Step Fix
1. Verify every landline telephone plugged into wall jacks uses an inline DSL microfilter.
2. Ensure the DSL modem itself is plugged directly into a non-filtered wall jack using a short RJ11 phone cord.
3. Check the DSL sync light; if blinking continuously, test the modem at the NID (Network Interface Device) box outside your house to isolate internal wiring shorts.
What happens when you connect a laptop DIRECTLY to the modem using an Ethernet cable (bypassing the third-party router entirely)?
- Laptop gets internet access immediately when plugged into modem, but router gets NOTHING
- Laptop ALSO gets no internet access when plugged directly into modem
ISP CMTS Locked to Previous Device MAC Address
Solution:
Diagnostic Cause
ISPs assign a single public IP address via DHCP to the exact MAC address of the first device connected to the modem. Plugging a new router in while the modem is powered on causes the ISP network to block the new router's WAN MAC.
Step-by-Step Power Sequence Fix
1. Turn off or unplug BOTH the modem and the router.
2. Disconnect the Ethernet cable running between modem and router.
3. Wait 10 minutes (this forces the ISP DHCP server to release the bound MAC lease).
4. Plug in the modem power cord FIRST.
5. Wait 3–5 minutes until all modem signal lights (DS, US, Online) are solid green.
6. Connect the Ethernet cable from the modem's LAN Port 1 into your router's WAN / Internet Port.
7. Plug in your router's power cord and wait 2 minutes.
What type of connection requirements does your ISP use?
- Standard Cable/Fiber DHCP (Dynamic IP), but ISP binds to a specific registered MAC address
- Fiber / DSL requiring a Username and Password (PPPoE Authentication)
- Fiber provider requiring VLAN ID Tagging (e.g., CenturyLink, Telekom, Chorus, Spark)
ISP Authorization Locked to Original PC/Gateway MAC
Solution:
Step-by-Step Fix
1. Connect your PC to the router via Wi-Fi or LAN.
2. Log into your router's web admin console (e.g., 192.168.0.1 or 192.168.1.1).
3. Navigate to Network > WAN or Internet settings.
4. Locate the MAC Clone section.
5. Click Clone Computer MAC Address (or manually input the MAC address of the laptop that successfully connected directly to the modem).
6. Click Save/Apply and reboot the router.
Missing PPP Authentication Handshake
Solution:
Diagnostic Cause
DSL and many fiber optic ISPs do not assign WAN IPs automatically via standard DHCP. They require point-to-point authentication credentials.
Step-by-Step Fix
1. Log into your router admin console.
2. Navigate to Internet / WAN Settings.
3. Change WAN Connection Type from *Dynamic IP (DHCP)* to PPPoE.
4. Enter your ISP-provided Username (often user@isp.net) and Password.
5. Set Internet Connection Mode to Always On.
6. Click Save and verify if WAN IP populates.
Un-tagged WAN Traffic Dropped by Fiber ONT Switch Port
Solution:
Diagnostic Cause
Direct fiber installations require internet traffic exiting your router's WAN port to be tagged with a specific 802.1Q VLAN ID (e.g., VLAN ID 201 for CenturyLink, VLAN ID 10 for Spark/Chorus, VLAN ID 7 for Deutsche Telekom). Without this tag, the ONT drops all packets.
Step-by-Step Fix
1. Log into your router admin console.
2. Navigate to Advanced > IPTV / VLAN or WAN Settings.
3. Enable VLAN / IPTV Mode.
4. Select your ISP from the preset list, or choose Custom.
5. Set Internet VLAN ID to the exact integer required by your provider (e.g., 201).
6. Set 802.1Q Priority to 0 (or as specified by ISP).
7. Apply settings and reboot.
Log into your router's status dashboard (`192.168.1.1` or `192.168.0.1`). What number appears under the **WAN IP Address** field?
- WAN IP shows '0.0.0.0' or 'Connecting...'
- WAN IP starts with 192.168.x.x, 10.x.x.x, or 172.16.x.x (Private/Double NAT)
- WAN IP starts with 100.64.x.x to 100.127.x.x (Carrier-Grade NAT)
- WAN IP is a valid public IP (e.g., 24.x.x.x or 73.x.x.x), but websites fail to load
Double NAT / Two Competing DHCP Servers on Network
Solution:
Diagnostic Cause
If your ISP provided a combination Modem/Router Gateway and you plugged your own secondary router into it, both devices are running NAT and DHCP servers. This creates IP routing loops that disconnect internet access.
Step-by-Step Fix
1. Connect a PC directly to your ISP gateway modem.
2. Log into its admin interface (typically 192.168.0.1 or 192.168.1.254).
3. Locate Wireless Settings and turn off Wi-Fi.
4. Locate Routing Mode or Bridge Mode settings.
5. Switch device operation from *Gateway / Router Mode* to Transparent Bridge Mode.
6. Connect Ethernet cable from Modem Port 1 into your standalone router's WAN port.
ISP CGNAT (RFC 6598) Dynamic Address Translation Exhaustion
Solution:
Diagnostic Cause
An IP address in the 100.64.0.0/10 block indicates your ISP uses Carrier-Grade NAT (CGNAT) to share a single public IP among hundreds of customers. Port forwarding and dynamic routing handshakes will fail.
Action Required
1. Call your ISP customer service line.
2. Request a Static Public IPv4 address or ask them to opt your account out of CGNAT.
3. Ensure IPv6 is enabled in your router's WAN settings (many CGNAT ISPs route IPv6 natively without address exhaustion).
ISP DNS Server Blackhole / WAN Packet Fragmentation
Solution:
Diagnostic Cause
Your router holds a valid public WAN IP, but the DNS servers assigned automatically by your ISP are failing to resolve domain names, or the Maximum Transmission Unit (MTU) packet size is causing packet fragmentation drops.
Step-by-Step Fix
1. Log into your router admin console.
2. Go to Network Settings > WAN / Internet.
3. Change DNS assignment from *Get Dynamically from ISP* to Use Custom DNS:
Primary DNS: 1.1.1.1 (Cloudflare)Secondary DNS: 8.8.8.8 (Google)4. Locate MTU Size settings (Maximum Transmission Unit):
For standard Cable/Fiber DHCP connections, set MTU to 1500.For PPPoE connections, set MTU to 1492 (or 1480 if stability issues persist).5. Click Save, clear your computer browser cache, and test internet connectivity.