Windows7: Troubleshooting Networking from the Command Line (part 2)

Checking Connectivity with the PING Command

As you might know, a submarine can detect a nearby object by using sonar to send out a sound wave and then seeing whether the wave is reflected. This is called pinging an object.

Windows 7 has a PING command that performs a similar function. PING sends out a special type of IP packet—called an Internet Control Message Protocol (ICMP) echo packet—to a remote location. This packet requests that the remote location send back a response packet. PING then tells you whether the response was received. In this way, you can check your network configuration to see whether your computer can connect with a remote host.

To use PING, first open a command-line session by selecting Start, All Programs, Accessories, Command Prompt. Here’s a simplified version of the PING syntax:

ping [-t] [-n count] target_name

Here’s an example that uses PING on the Google.com domain:

C:\Users\Paul>ping google.com


Pinging google.com [64.233.187.99] with 32 bytes of data:

Reply from 64.233.187.99: bytes=32 time=43ms TTL=240
Reply from 64.233.187.99: bytes=32 time=42ms TTL=239
Reply from 64.233.187.99: bytes=32 time=43ms TTL=239
Reply from 64.233.187.99: bytes=32 time=42ms TTL=240

Ping statistics for 64.233.187.99:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),A
Approximate round trip times in milli-seconds:
    Minimum = 42ms, Maximum = 43ms, Average = 42ms

Here, you see that each echo packet received a reply. If you can’t connect to the remote host, PING returns a Request timed out message for each packet.

If you can’t connect to a remote host, here are some notes on using PING to troubleshoot problems:

• First, check to see whether you can use PING successfully on the loopback address:

  ping 127.0.0.1.

  The only reason this PING would fail is if your computer doesn’t have the Internet Protocol installed. However, all Windows 7 machines have IP installed, and the option to uninstall it is disabled, so pinging the loopback address will almost certainly work. The only reason to include it in your troubleshooting is that if it doesn’t work, it means you have a serious problem with your machine.

• Try using PING on your computer’s IP address. (If you’re using DHCP, run the IPCONFIG utility to get your current IP address.) If you don’t get a successful echo, your NIC may not be inserted properly or the device drivers may not be installed.
  

• Now PING another computer on your network. If PING fails, check your cable or wireless connections.

• The next test you should run is on your default gateway (that is, your router). If you can’t successfully PING the router’s internal IP address, you won’t be able to access remote Internet sites. In this case, check the IP address you entered for the gateway, check the cable connections, and make sure the router is turned on. You may need to power cycle the router.

• If you get this far, try using PING on the remote host you’re trying to contact. If you’re unsuccessful, check to make sure that you’re using the correct IP address for the host. Try power cycling your broadband modem.

Tracking Packets with the TRACERT Command

If you can’t PING a remote host, it could be that your echo packets are getting held up along the way. To find out, you can use the TRACERT (trace route) command:

tracert [-d] [-h maximum_hops] [-j host-list] [-w timeout] target_name

TRACERT operates by sending ICMP echo packets with varying TTL values. Recall that TTL places a limit on the number of hops that a packet can take. Each host along the packet’s route decrements the TTL value until, when the TTL value is 0, the packet is discarded (assuming that it hasn’t reached its destination by then).

In TRACERT, the ICMP packets specify that whichever host decrements the echo packet to 0 should send back a response. So, the first packet has a TTL value of 1, the second has a TTL value of 2, and so on. TRACERT keeps sending packets with incrementally higher TTL values until either a response is received from the remote host or a packet receives no response. Here’s an example of a TRACERT command in action:

C:\>tracert google.com


Tracing route to google.com [216.239.57.99]
over a maximum of 30 hops:

  1    <1 ms    <1 ms    <1 ms 192.168.1.1
  2     8 ms     8 ms     8 ms 64.230.197.178
  3     6 ms     6 ms     6 ms 64.230.221.201
  4     6 ms     6 ms     6 ms 64.230.234.249
  5     8 ms     6 ms     7 ms 64.230.233.93
  6    17 ms    17 ms    16 ms core1-chicago23-pos0-0.in.bellnexxia.net [206.108.103.130]
  7    17 ms    17 ms    17 ms bx2-chicago23-pos11-0.in.bellnexxia.net [206.108.103.138]
  8    17 ms    17 ms    17 ms so-4-3-3.cr1.ord2.us.above.net [208.184.233.185]
  9    18 ms    17 ms    18 ms so-0-0-0.cr2.ord2.us.above.net [64.125.29.186]
 10    36 ms    36 ms    36 ms so-5-2-0.cr1.dca2.us.above.net [64.125.30.225]
 11    47 ms    46 ms    46 ms so-4-1-0.mpr2.atl6.us.above.net [64.125.29.41]
 12    48 ms    48 ms    48 ms 64.124.229.173.google.com [64.124.229.173]
 13    48 ms    48 ms    48 ms 216.239.48.23
 14    49 ms    49 ms    49 ms 216.239.46.44
 15   100 ms   100 ms   100 ms 216.239.47.129
 16    99 ms    99 ms    99 ms 216.239.49.250
 17    99 ms    99 ms    99 ms 66.249.95.65
 18    99 ms    99 ms    99 ms 66.249.94.27
 19   102 ms   101 ms   101 ms 216.239.49.97
 20    99 ms   100 ms    99 ms 216.239.57.99

Trace complete.

					  

Changing the Default TTL Value

One of the reasons your packets might not be getting to their destination is that the default TTL value used by Windows 7 might be set too low. This is actually very unlikely because the default is 128, which should be more than enough. However, you can increase this value if you want. Start the Registry Editor and highlight the following key:

HKLM\System\CurrentControlSet\Services\Tcpip\Parameters

Select Edit, New, DWORD Value, type DefaultTTL, and press Enter. Change the value of this new setting to any decimal value between 0 and 255 (0 to FF in hexadecimal).