Share Internet Connection Over RJ45 to a Modem with Phone Line Input

Abstract: Resolves the inability to share internet from a computer directly into a modem by explaining proper router setup and Windows Internet Connection Sharing.

Created: 2026-10-14 by UserComp.com Editors

You cannot directly share an internet connection from a computer to a modem via RJ45 because modems are designed to receive signals from the Internet Service Provider (ISP), not to act as network clients. The correct approach is to use a router between the modem and your devices, or configure a Windows PC with two network adapters to bridge the connection for other devices on the same local network.

Understanding Modem vs Router Functionality

The confusion often stems from mixing up the roles of networking hardware. A modem (modulator-demodulator) connects to your ISP’s infrastructure—whether via coaxial cable, fiber optic line, or telephone jack—and translates that signal into digital data. It typically has only one Ethernet port intended for upstream communication with the ISP’s gateway. You cannot plug a standard Ethernet cable from a computer into this port to "feed" the modem internet; the modem expects the internet source to be on its WAN side, not its LAN side.

In contrast, a router creates a local area network (LAN). It takes the single internet connection from the modem and distributes it to multiple devices via Wi-Fi and additional Ethernet ports. If you are trying to share a connection from one device to another without a dedicated router, you must use software bridging features available in modern operating systems like Windows 10 or 11.

Method 1: Using a Dedicated Router (Recommended)

This is the most stable and secure method for sharing internet access. Most users who ask this question actually need to connect their existing modem to a router, which then shares the connection to their PC or other devices.

Connect the Modem to the Router: Take an Ethernet cable and plug one end into the Ethernet port of your modem. Plug the other end into the WAN or Internet port on your router. This port is usually color-coded differently (often blue or yellow) than the LAN ports.
Power Cycle the Devices: Turn off both the modem and the router. Wait 30 seconds. Turn the modem back on first and wait for all lights to stabilize. Then, turn on the router.
Connect Your Computer: Use another Ethernet cable to connect your PC to one of the numbered LAN ports on the router (e.g., LAN 1, LAN 2). Alternatively, connect via Wi-Fi using the network name and password found on the router’s label.
Verify Connection: Open a web browser and navigate to any website. If the page loads, your router is successfully receiving the internet from the modem and distributing it to your computer.

If you are setting up a complex network with multiple routers, see how to configure a modem to support multiple computers via Ethernet for advanced bridging techniques.

Method 2: Internet Connection Sharing (ICS) on Windows

If you do not have a physical router and want to share your PC’s internet connection (from Wi-Fi or a cellular USB adapter) with another device via Ethernet, you can use Windows Internet Connection Sharing. This effectively turns your PC into a software-based router.

First, ensure your PC has two active network connections: one that has internet access (source) and one connected to the device you want to share with (target). For example, your PC might be connected to Wi-Fi (Source) and have an Ethernet cable plugged into a second PC or a small switch (Target).

Navigate to Control Panel > Network and Sharing Center > Change adapter settings. Right-click the adapter that has the internet connection (e.g., Wi-Fi) and select Properties. Go to the Sharing tab. Check the box that says Allow other network users to connect through this computer’s Internet connection. In the dropdown menu below, select the Ethernet adapter that is connected to the target device. Click OK.

Windows will automatically assign the target device an IP address in the 192.168.137.x range. The target device should now be able to access the internet through your PC. Note that this method requires your PC to remain powered on and connected to the source internet at all times.

Troubleshooting Common Issues

Even with proper hardware setup, connectivity issues may arise. First, check your physical cables. Ensure the Ethernet cable is securely seated in both the modem/router and the computer. Try a different cable if possible, as damaged wires can cause link failures.

If you have a router but no internet access, log into the router’s admin panel by entering its default IP address (commonly 192.168.0.1 or 192.168.1.1) into a web browser. Verify that the router has obtained a public IP address from the modem. If it shows 0.0.0.0 or an APIPA address (169.254.x.x), there is a communication failure between the modem and router.

For persistent lag or disconnection issues after setup, refer to our guide on troubleshooting Wi-Fi lag on ISP routers, as these steps often apply to wired connections as well when NAT or DHCP settings are misconfigured.

Hardware Compatibility Considerations

When selecting equipment, ensure compatibility between your modem and any third-party routers. Some ISPs lock their modems to specific MAC addresses or require specific firmware configurations. If you replace an ISP-provided modem/router combo with a standalone modem and a third-party router, you may need to clone the MAC address of the old device or contact your ISP to register the new router’s MAC address.

Additionally, verify the speed capabilities of your cables and ports. Older Ethernet cables (Cat 5) support up to 100 Mbps, while Cat 5e and Cat 6 support Gigabit speeds (1000 Mbps). Using outdated cabling can bottleneck your internet speed regardless of your plan. See the differences between 10 Mbps, 100 Mbps, and 1000 Mbps BASE-T standards to understand how physical layer differences impact performance.

Frequently Asked Questions

Can I plug my PC directly into the phone line jack of a DSL modem?

No, you cannot plug an Ethernet cable from a PC directly into a phone line jack. DSL modems require a specific DSL filter and use telephone wiring protocols, not Ethernet. You must connect your PC to the Ethernet port on the modem, which then connects to a router, or use a USB-to-Ethernet adapter if supported by the modem manufacturer.

Why does my router show “No Internet” even though the modem lights are green?

This usually indicates a configuration issue rather than a physical failure. Check if the router’s WAN port is correctly set to obtain an IP address automatically (DHCP). Sometimes, the modem needs to be rebooted after connecting the router to establish a fresh lease on the IP address from the ISP.

Is Internet Connection Sharing (ICS) safe for my home network?

ICS provides basic NAT protection, which hides your internal network from the outside world, but it lacks the advanced firewall features of a dedicated consumer router. It is suitable for temporary setups or trusted devices on a local network, but for long-term security and parental controls, a dedicated hardware router is strongly recommended.

What happens if I connect two routers together incorrectly?

If you connect two routers via their WAN ports, they create separate subnets, which can cause double NAT issues and prevent devices on one network from seeing devices on the other. If you connect them via LAN-to-LAN ports, you must disable DHCP on the second router to avoid IP conflicts. Always consult how to properly configure a router as a DHCP server to avoid these conflicts.

Can I use a wireless repeater instead of an Ethernet cable to share the connection?

Yes, a wireless repeater or extender can expand your Wi-Fi network, but it does not provide a wired Ethernet output to a modem. If you need to connect a device that only has an Ethernet port, such as a smart TV or a desktop PC, a dedicated router or the ICS method described above is necessary.