Internet functions via the router but fails when linked directly to the modem.
Internet functions via the router but fails when linked directly to the modem.
This situation seems unusual. My connection was strong until recently, then dropped to around 150-200 Mbps while upload stayed normal. The modem’s downlink light stopped working. I tried standard fixes and even had a technician check it out, but they found no device could reach the internet when plugged directly into the modem. After reboots and resets, nothing changed. When connected to the router, speeds returned but were slower. We’re both confused, and the only suggestion was replacing the whole modem. I bought this one about a year ago after ongoing issues with my ISP’s equipment, so I’d rather not replace it unless it’s clearly faulty. I’m still using the Motorola MB8600 and a Dream Machine Pro. The modem is under warranty for a short time, but I don’t want to deal with more trouble if it’s actually working fine and there’s something obvious missing. I’m open to sharing any data you need for troubleshooting. Thank you for your help!
It makes sense that the direct link failed since your PC and the ISP tech didn’t match. The ISP’s system appears to route from the exchange to a specific MAC address, but your device has a different one, so connection is blocked. Your router is still covered by warranty—consider replacing it to resolve the issue. Other possibilities include a power disruption or throttling due to high usage. Double-check if you accidentally exceeded data limits. Resetting the modem might also help.
The issue might be with the DNS configuration on your modem, as it isn't correctly providing the necessary settings for your clients. Your router likely has its own DNS setup and is functioning properly, so clients are using that instead. Try changing the DNS to 1.1.1.1 and observe if the problem resolves.
The ISP focuses on the modem's MAC address. Usually the modem and its serial number are provided to the ISP so they can register the device. In this case, the Motorola MB8600 is the only device the ISP recognizes in the user's home because it's the authorized one. The MAC address of the user's PC compared to their router would have led the modem to request a different IP from the ISP's DHCP server, yet the connection should have worked.
I've noticed this often: when the modem doesn't assign a new IP, it's because the device's MAC address is still registered. A reboot usually clears that issue and restarts the DHCP lease. Generally, the ISP only needs the modem's MAC or serial number for authentication to their network.
You're interpreting your ISP's behavior as being focused solely on the modem's MAC address rather than the actual device connected. They likely prioritize the hardware identifier over the user's application. Technically, you have two modems—one for internet and another for phone—each handling different traffic types. Connecting your computer to the second modem worked fine, suggesting it received a valid IP address. Reading about modems assigning a single IP on startup might explain why they didn't recognize a new device without a reboot. Despite restarting and resetting your setup, you're still facing issues. Your equipment is protected by power supplies and surge protection, so outages won't affect you. Although you consume significant data—around 5TB monthly as a video editor—you haven't experienced throttling before, making it hard to pinpoint the cause. Paying for unlimited data doesn't guarantee immunity from these problems, as past experiences show.
Docsis (Cable Internet) allows you to distribute the available bandwidth among connected devices within the same cable connection. There might also be some interference present. Is your modem equipped with a web interface that shows logs or a spectrum analyzer? Also, have you observed anyone entering or leaving your building/street/area recently? Your question stems from knowing an older student dorm that used cable until Ethernet was installed around the 2000s. That connection was frequently disrupted whenever a foreign student moved in with a TV from Japan. Their equipment uses slightly different specifications compared to ours, which can cause interference similar to how a wireless access point from another region operates at different power levels.
I struggled to get the modem to connect any directly connected devices, but I managed to fix the speed problems, which was satisfactory. My UDM Pro had numerous IPS traffic-blocking rules that were likely causing the issue. I used a Plex server with port forwarding, but stopped it recently because I kept receiving frequent "network intrusion attempt" alerts. I enabled intrusion prevention and blocked individual IPs or ranges when multiple requests came from similar addresses. Over time, the list became extensive and didn’t stop incoming traffic, so I turned off port forwarding to reduce spam. I also forgot to remove unnecessary rules, which led to significant bottlenecks and high latency. All those restrictions are now disabled, restoring normal internet performance. Thanks for your assistance!