If you are having trouble reaching services on the Internet (e.g., email, web sites, game servers), it's a good idea to first try at least one quick LAN ping test (from the computer to the router) as shown in Test 5: The Handy-Dandy LAN Ping Test. Once you have checked and passed the LAN ping test, a good approach is to first ping things relatively "close" from a network point of view and then ping IP addresses further and further away. This helps isolate if the problem is in your network, your ISP's network or even further out.
The first step is to ping the gateway address that your router is using to communicate with the rest of the Internet. If you have a fixed IP address, this is relatively simple because your ISP will have told you what your gateway address is when they assigned you your IP address. Open a command window and type ping followed by your WAN gateway address as in the screen below.
If you have a dynamically assigned IP address, determining your gateway address can be a bit trickier, but your router will likely give you a hand. First, you first need to find out if your router has successfully retrieved an IP address from your ISP's DHCP server. Your router's status page should list your current external IP address. The follow screen capture illustrates that case.
Recall that for the purposes of demonstrating the various problems that can occur, I have set up a test network inside my real home network. This is why the IP address I received is another non-routable (private) IP address. The IP address retrieved was 192.168.3.41 in this case. (See Private IP Addresses for more details on private IP addresses.) When you look at the status page for your router, it will likely be a public IP address like 162.84.93.33. The gateway address was also retrieved at the same time, which is 192.168.3.1. Most routers will display the gateway address they received in the DHCP response. Substitute that IP address in the ping command shown above. If it doesn't display the gateway explicitly, try using the first three octets (e.g., 162.84.93) and end with a .1 (for a full IP address of 162.84.93.1. In the command window above, the ping to 162.84.93.1 is a ping to my ISP's WAN gateway for a real IP address. If your ping test is successful, skip to the next step, Baby Step 2: Ping Your ISP's DNS Server. If not, proceed into the following troubleshooting section.
If you aren't able to successfully ping your ISP's gateway address (or you aren't getting a valid IP address returned from a DHCP request), the problem is generally isolated to the connection between your router and DSL/cable modem or the modem's connection to the ISP. It's also possible your ISP may be having a temporary (planned or unplanned) outage.
Log into your router and browse to its status page. (If you are unable to log into your router, check to make sure it has power. Also, try to ping the router's LAN IP address as was done in Test 5: The Handy-Dandy LAN Ping Test.) If your router shows an Internet address that is not valid (like 0.0.0.0 in the screenshot below) most likely you've lost connectivity with your ISP. (That's assuming you had connectivity before. If your high-speed connection is a new installation, it may not be active yet.) The first thing to check is the power on the modem and Internet/WAN lights on the router and modem.
On the example router shown here, the Netgear WGT624, the power indicator is the light to the far left and the light that is second from the left, which is a lowercase letter "i" with a circle partway around the bottom, is the WAN/Internet port. They are green when the router has power and the connection to the DSL or cable modem is correct, respectively. If the power light is not lit, check your power adapter and power connection on the back of the unit to make sure they are plugged in. If it is, try moving it to another plug. (Use an extension cord if it's inconvenient to move the router.) I've have a number of those little power adapter "bricks" die, so don't be surprised to find out that the router won't power up even when you try a working outlet. (Radio shack has substitues for these with variable DC voltage outputs and tips that will work with all sorts of devices. Check to make sure the amperage is sufficient.)
Next, check the corresponding lights on your Cable or DSL modem. On the example cable modem shown to the right, the power light is at the top and labeled "Power" and the LAN light is labeled PC/Activity and is the second light from the bottom. Finally, check that your connection to your ISP is established.
On most DSL modems I have used, there is a single light labeled Internet, Ethernet, WAN, Connection or something similar. It should be lit and steady when the connection is established to your ISP but no traffic being sent or received. The light will blink or flicker when Internet network activity is occurring in either direction. Note the Ethernet light is on in the picture below.
If the Ethernet light is not lit, it's time to start suspecting your service from your ISP is down. First, make sure that the telephone line from the DSL modem to the wall is secure. You may want to try another telephone wire just to be sure. Also, check to see if your regular phone service is working. If both your DSL and telephone service are out, your line may have been cut or disconnected. That's a problem for your phone company to deal with. If the problem lies just in your DSL connection, you should call your ISP's technical support and see if there is a general problem they are aware of, if they are doing some planned upgrades or maintenance they didn't notify you about, and if you are the only person with an outage and need to report it.
On the particular cable modem shown here, the Motorola SB5100, three lights are used to show you the status of your cable connection to your ISP. The first light (second from the top under the Power light) is labeled "Receive" and is the receive channel established light. It is lit when a downlink (incoming) connection from your ISP is established. The next light down is labeled "Send" and is lit when an uplink (outgoing) channel to your ISP has been established. If either the Send or Receive light is not lit, it's probable that your cable service is down or the cable is broken or disconnected. The next light is labeled "Online" and indicates all necessary negotiation such as DHCP address assignment has completed. When this light is not lit, it generally means that the DHCP negotiation failed either initially during power on or after a DHCP IP address lease has expired and a new IP address could not be retrieved from your ISP's DHCP server. If your cable appears to be in working order, you should call your cable Internet service provider to find out is a general problem they are aware of, if they are doing some planned upgrades or maintenance they didn't notify you about, and if you are the only person with an outage and need to report it.