CenturyLink Speed Test
Use this tool to check your internet speed for both CenturyLink DSL and Quantum Fiber connections. Accurate testing helps identify if your modem or line quality is limiting your performance.
CenturyLink provides two main types of service: legacy DSL (using copper phone lines) and the newer Quantum Fiber (using 100% fiber optics). Your expected results will vary significantly between these technologies.
Interpreting Your Results
Quantum Fiber: You should expect symmetrical speeds (e.g., 940 Mbps Download / 940 Mbps Upload). If your upload is low, check your ethernet cables.
DSL: Speeds are distance-dependent. Expect downloads between 10-140 Mbps and uploads significantly lower (often 1-20 Mbps). This asymmetry is normal.
CenturyLink C4000 Modem Series
Most customers use the "Greenwave" C4000 series (C4000LG, C4000XG, C4000BG). Here are the key details:
| Modem Feature | Details |
| Login IP | 192.168.0.1 |
| Username | admin |
| Password | Found on the sticker (NOT your Wi-Fi password) |
Understanding Modem Lights (Ring Light)
The C4000 series uses a single LED ring to communicate status:
- Solid Green: Connected to the internet. Everything is working.
- Solid Blue: Booted up and ready for setup (or sometimes "Connected" on specific fiber firmware).
- Blinking Blue: Searching for connection. This happens during boot-up.
- Amber / Orange: "Walled Garden" state. You need to complete setup in the browser or app.
- Red: Connection failure. Check your cables.
Troubleshooting Slow Connections
1. Check for "Bridged Mode"
If you have your own router connected to the CenturyLink modem, you should enable Transparent Bridging on the CenturyLink modem. This disables its router functions.
Note: For Quantum Fiber, your personal router may need to support VLAN Tagging (VLAN 201) on the WAN port to work directly with the ONT.
2. DSL Line Noise
For DSL users, crackling on your home phone line indicates interference. Ensure every phone jack has a DSL Filter installed, except the one plugged into the modem.
3. Wi-Fi Congestion
The C4000 series supports Wi-Fi 6. Ensure you are using the 5GHz band for speed tests. The 2.4GHz band is often congested and slow.
How to Login to CenturyLink Modem
- Ensure you are connected to the modem's network.
- Open a web browser and enter 192.168.0.1.
- Enter the username admin.
- Enter the Admin Password from the sticker on the modem base.
- Click "Apply" to access settings like SSID, Port Forwarding, or DMZ.
When to Contact Support
Call CenturyLink/Quantum Fiber support if:
- Your modem light stays Red for more than 15 minutes.
- Your DSL line has audible static that filters don't fix.
- Your Fiber speed test is consistently below 50% of your plan speed on a wired connection.
CenturyLink & Quantum FAQs
Why is my CenturyLink modem light blinking blue? A blinking blue light means the modem is searching for the ISP signal. If it persists, check the cable connection to the wall.
What is the difference between CenturyLink and Quantum Fiber? Quantum Fiber is the brand name for the new fiber-optic network (formerly CenturyLink Fiber). It offers superior reliability and speed compared to the legacy copper DSL network.
Can I use my own router with CenturyLink? Yes. Put the C4000 modem in Transparent Bridge mode. For Fiber users, you can sometimes verify if you can plug your router directly into the ONT (SmartNID) requires VLAN 201 tagging.
Alternatives to check
If CenturyLink speeds aren't meeting your needs, compare with:
- Cox Speed Test: Often the cable alternative in CenturyLink markets.
- Xfinity Speed Test: Major competitor in many cities.
- Starlink Speed Test: Good for rural areas where DSL is slow.