Ethernet cable functions properly in the router yet fails in the modem.
Ethernet cable functions properly in the router yet fails in the modem.
Hi, I set up two Ethernet cables from my modem to two desktops. One works fine, but the other shows "Ethernet doesn’t have a valid IP." The issue disappears when plugging it into the router. I’m trying to figure out how to get both devices to connect properly through the modem. I’ve searched online but haven’t found a clear solution. Thanks!
Plugging the cable into the router triggers the device to recognize the new network card at the end of the connection. The router’s built-in DHCP server then assigns an IP address to that card. When using a modem, the setup might be set to pass-through mode, meaning the modem forwards the connection without changing the IP. Alternatively, the modem could be limited to providing a single IP address, which it assigns to the router or the first computer, and then blocks any further IP requests.
Most coax/cable modem suppliers provide just one public IP address. Restarting typically removes the initial allocation, allowing a fresh one to appear for another device such as a router.
The confusion often arises because modems, gateways, and routers are frequently mixed up. They serve different purposes. 1) A modem converts internet service from a cable, phone line, or coax into Ethernet. It doesn’t offer firewalls, NAT, Wi-Fi, or a network switch. You need a router if you want several devices connected, especially when your ISP assigns only one IPv4 address. Cable modems with many ports are mainly for higher speeds. Docsis 3.1 supports up to 10 Gbps down and 2 Gbps up, which requires multiple Ethernet ports. But with newer Ethernet standards of 2.5 and 5 Gbps, those modems may become less common. 2) A router connects to a modem and lets you link multiple devices using NAT. It provides security features like firewalls, DHCP, and often includes a built-in network switch. 3) A gateway combines a modem and a router into one unit. Fiber connections typically don’t need a modem; they use a media converter to switch between fiber and copper (like Ethernet or coax). Sometimes ISPs include a gateway with a router inside. The important thing is understanding what you have. As @mariushm noted, you might find a gateway operating in bridge or passthrough mode, turning it into a standard modem or converter if you have fiber.
I’m not sure why you’re seeing this behavior with your setup. With four cables connected to the same fiber port, it seems like the main desktop can access all ports directly, while the other desktop relies on the router for connectivity. This might be due to how the network is configured or the device settings. Hope this helps clarify things!
This doesn’t seem logical to me… if your IPV4 they’re not providing you three IP addresses? And with IPV6… they shouldn’t restrict how many you receive at all since IPV6 allows each device its own public IP. In any case, likely what you should do is set the modem to pass-through mode, switch it to DMZ, or connect a router between them and route everything through it.
It seems the initial tech help might be inadequate or unmotivated. Your ISP controls the device, which explains why you’re connected now. They manage the authorization and update the firmware regularly.