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Ask LH: Why Does My Modem Blink Even When I'm Not Online?

Dear Lifehacker, My modem and router keep flashing all night, even when I’m not doing anything on the internet. I know my power light is supposed to be on, but should the send/receive light be blinking even when all my computers are turned off? Am I using up data? Help! Sincerely, Blinking Lights

Dear Blinking Lights,

First of all, there’s probably no reason to worry about this. There are a number of normal things happening in the background on your home network, out across the internet at large, and between your modem and your ISP that can make it look as if there’s a lot of traffic even when all of your computers and equipment are turned off.

We’re assuming the lights you’re concerned with are the send/receive or “traffic” lights — not the power, uplink or PC connection lights. Power should be on if the device is plugged in, uplink should be on and steady if you have a connection to your ISP, and the PC connection light should be on whenever there’s an ethernet cable to a computer or router. Remember, the send/receive light on your modem or router is an indicator of activity, not of volume. Here’s what might be going on:

Your ISP Is Checking That Your IP Address Is In Use

Even if all of your equipment is turned off and unplugged from your cable modem or router, you will still see some traffic coming from your ISP. Most frequently, your ISP is sending your modem ARP requests, or attempting to see if the IP address it has assigned to you via DHCP is still in use. If it is, your ISP won’t touch it. If it detects your modem as being offline, it will assign that address to its block of available addresses to give out to users coming online who need IP addresses. How frequently that process takes place varies from ISP to ISP, but most large service providers do this automatically and very frequently. Picture: shaymus22/Flickr

Your ISP is Checking That Your Connection Is Healthy

Another reason those lights are flashing all the time is that your ISP may be checking to see if your connection is healthy — regularly scanning online modems and gateways to make sure everything is alive and running properly. ISPs do this less frequently than scanning for available IP addresses to retrieve, but it’s another reason why you may see some traffic even if all of your computers or routers are turned off. Again, it’s just another way ISPs keep an eye on the health of their networks.

You’re Being Port-Scanned by Botnets (But Don’t Worry!)

Don’t panic, but a large portion of the traffic you’re seeing, even when your computers are turned off and your router is disconnected, is probably your IP address and network being blindly port-scanned by botnets or other potential attackers who are looking for systems with known vulnerabilities they can easily exploit. This has been true for years, and there are more would-be attackers around now than there used to be.

But we mean it when we say not to panic: if the first thing you do when you put a new computer on the internet is download the latest updates for it, you’re usually in good shape. You’re in even better shape when you’re using a computer with a firewall running, and a router with a built-in firewall (or, more accurately, does Network Address Translation, which almost all consumer routers feature these days). As long as your computer is up to date, and you’re safely behind a router or firewall, those random, sweeping port scans are nothing to worry about. There’s nothing you can do about them except practise good computer habits and hygiene.

Log In to Your Router And Check Your Connection

As long as you know what computers and wireless devices are using your internet connection, and you know they’re all off, then the reasons above can explain why your router or modem’s connection lights are flashing. If you’re still not sure though, it’s possible there’s an unknown device connected to your network. Your router could be the problem (in which case, you may need to troubleshoot it), or there may be another device on your network.

To check, Log in to your router (if you’re not sure how, our night school guide will get you familiar with your home network) and check to see which devices are currently online. Almost every router lets you see a list of computers, tablets, smartphones and other devices connected to the network. You may only see them listed by IP address or MAC or hardware address, but set-top boxes like Apple TVs are often noted as such. Most modern routers will even tell you the computer’s network name.

Smartphones and tablets are a little trickier to identify, but a quick look at the device itself should tell you its IP address or MAC address (here’s how to find it on iOS and on Android) and you can match it up with what you see in your router’s Admin page. If you see something out of the ordinary, you can always boot it from the network and wait to see if someone in your house complains about getting disconnected. There could be a tablet or set-top box upstairs that is online when you thought it wasn’t. In the worst case scenario, someone else may be using your wireless network behind your back. If that’s the case, it’s time to change your Wi-Fi password and your router’s admin password.

Check Your Computer To See What’s Being Communicated

Sometimes it might not be a device on your network making your modem’s lights flash — it could be an application on one of your computers. Even when all of your applications are closed, there may be a service or application running in the background communicating with the internet to make sure it has the most up-to-date version of itself. To find out on a Windows machine, try a standard firewall like Comodo Firewall or an app like NetLimiter to uncover which applications are talking to the internet and when. Mac users can use Little Snitch or previously mentioned Radio Silence to see what applications are talking to the web. Then you can block or allow them as you see fit.

In most cases, flashing lights on your router are a good thing. They indicate your connection is up and healthy. Just because your computers are off or your router is unplugged doesn’t mean that those lights should stop flashing altogether. As long as you keep your computers safe and protected, those lights are more than likely indicators that everything is running normally.

Cheers
Lifehacker

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