Print this page while your connection is working and save it for the day that it fails. These instructions will be helpful for troubleshooting any problems with any type of internet connection, dialup, DSL or Cable.
This first part applies to a computer/router/modem/internet connection, but if you have something else you can still adapt much of this page to your connection.
The easiest thing to try is to turn off (or unplug) the modem and router for a minute, then turn the modem back on and let it synch up. Once it's lights get back to normal, plug in the router and let it synch. You may need to reboot the computer after that depending on your operating system. See if everything works OK. If not, try the steps below. The information you get from this can also be helpful when you call your ISP if you need to.
What you need to do is determine where the failure is. There are several possibilities. ISP problems, modem problems, router problems, DNS problems, networking problems, or specific software (browser, email program) problems. These steps will help you determine which it is.
Open a command prompt and type "ping Yahoo.com" (without the quotes) and press Enter. You should see something like:
D:\>ping yahoo.com
Pinging yahoo.com [66.94.231.99] with 32 bytes of data:
Reply from 66.94.231.99: bytes=32 time=30ms TTL=52
Reply from 66.94.231.99: bytes=32 time=30ms TTL=52
Reply from 66.94.231.99: bytes=32 time=20ms TTL=52
Reply from 66.94.231.99: bytes=32 time=20ms TTL=52
Ping statistics for 66.94.231.99:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 20ms, Maximum = 30ms, Average = 25ms
If that's what you get, your connection and DNS is working and you want to troubleshoot the specific program that's having trouble.
If the response is something like:
D:\>ping yahoo.com
Pinging yahoo.com [66.94.231.99] with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Ping statistics for yahoo.com:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Then you're able to connect to the ISPs routers and DNS servers, but there's a problem further upstream. The important thing here is that your computer was able to contact DNS servers to resolve the site name to an IP address. That means that your connection is OK at least as far as your DNS servers, probably your ISPs servers unless you've set up something different. Please note that some sites don't respond to a ping, so even if your connection is fine, you may get no response. Try pinging a couple more sites. As of the time of this writing yahoo.com and google.com respond to pings. If you still get no response, call your ISP.
If you get:
D:\>ping Yahoo.com Unknown host Yahoo.com
then you either have no connection or DNS problems. Try:
Ping 66.94.231.99
If that works it's DNS trouble, if it doesn't your connection is down and you need to call your ISP once you've used the tips on this page to eliminate anything at your end, like your router.
Log into the router and check the Status page to make sure that it has DNS servers listed. If it does, then check the settings on your computer.
One other thing you can try is using an alternate DNS server. This list is from Speedguide.net, if/when you can connect I'd suggest verifying that the IP addresses haven't changed. I actually recommend using alternate servers all the time. Some ISP provided DNS servers are notoriously flakey.
One other thing you can do to try to determine the point of failure is sequential pinging.
Ping your router first and see if it responds. If it does, ping your modem (if you know the address, mine is 192.168.100.1). If they both respond, then the problem is out in the ISPs area. You can also [maybe] log into the modem to confirm that it's working. I've had a few occasions where I could log into the modem, and it's status page told me it was unable to connect. Sometimes a power cycle fixed it, other times the problem was the ISP server that the modem needed to connect to.
Often when you call your ISP they want you to disconnect the router. If you can convince them that you can get past the router and to the modem, that may convince the person you're talking to that the router isn't the issue. I'm extremely leery of disconnecting my router.