Networking

Get IT Done: Troubleshoot network problems using basic TCP/IP commands

Find out how the PING, TRACERT, and IPCONFIG commands can be used to quickly pinpoint a break in communication

Most corporations’ networks either run the TCP/IP protocol as their primary network or protocol, or, at minimum, their stack contains TCP/IP for Internet connectivity. If this is the case in your IT shop, troubleshooting your network problems using the built-in TCP/IP tool suite can save a lot of time—if you know the right tools to use.

PING
The Packet Internet Groper (PING) tool is very familiar to most IT pros. As with all TCP/IP utilities, it must be run from the command line. It is used to verify a connection to a remote computer, router, or printer (or anything that uses a TCP/IP address). The PING tool sends small packets of data to the remote system requesting a reply and then displays the results of the reply, as shown in Figure A.

Figure A


If a PING test shows a reply, then you know you have a physical link to the remote system. But if you get a result like the ones shown in Figure B, you know that there is a problem.

Figure B


The problem could be caused by a number of factors. Check to see that the remote system is not powered down, that there is not a piece of communication equipment between your system and a remote system that is down, and that you have the correct IP address. If none of these factors seems to be culprit, you’ll need to switch tools and use the TRACERT tool instead, which is discussed below.

Besides verifying TCP/IP connections, the PING command can also be used to test your DNS servers, as shown in Figure C.

Figure C


By pinging the friendly name of the remote location, your DNS servers should automatically resolve the friendly name to an IP address, as shown in Figure D.

Figure D


However, if your DNS servers are not responding, or the name you are trying to reach does not exist, you will receive a response like the one shown in Figure E.

Figure E


This lets you know that there is a DNS issue that needs further attention.

The PING tool can also allow you to check your network card. By pinging the IP address of 127.0.0.1, the command will send a PING request to your network card to see if it replies. Figure F is an example of what you will see if your network card is functioning and if TCP/IP is installed correctly.

Figure F


TRACERT
The next tool we will look at is the TRACERT (trace route) tool. The TRACERT tool shows the route taken by a packet of data to the destination IP address (or friendly name). It normally looks like Figure G.

Figure G


This shows that the packet you sent arrived at the destination location as well as every piece of communication equipment it passed through along the way.

If you cannot reach your destination address (which is usually checked with the PING tool), you can use the TRACERT tool to determine where exactly the break in communication is, as shown in Figure H.

Figure H


From this example, we know that the break in communication occurred on the 18th hop, right after reaching IP address 64.15.192.114. This shows that we must now begin looking at the last place the packet went to, where it was going from there, why it was sent to that address, if the address in the routing table is correct, and if there is a link between the sites.

IPCONFIG
The IPCONFIG or IP configuration tool shows the current TCP/IP information contained on the local machine, as shown in Figure I. This is useful (especially in DHCP environments) to see if the IP address of the local computer, the default gateway, and the subnet mask are correct.

Figure I


If you are in a DHCP environment and the client machine cannot connect to the network, one of the first steps you should take is to release and then renew the client IP address, as shown in Figure J.

Figure J


This does two things: First, it should clear any duplicate IP problems. Manually renewing the IP address will give you a clean IP from the DHCP database. If your DHCP lease has expired, it is possible that the same IP address has been issued to another machine on the network. If this happens, you will not be able to connect to the network until you have an IP address that is distinct on the network. When you renew your IP address, you should receive a unique IP address from the new DHCP database. Second, it will show you if there is an issue with your DHCP server. If you are unable to renew your IP address, then there is a good chance that there is an issue with your DHCP server’s database or network connectivity.

Another switch that can be used with the IPCONGIF tool is the /all switch, which displays the output shown in Figure K.

Figure K


This is all of the information for your computer’s current IP configuration. This screen is useful to see if your IP settings are configured correctly. Things such as an incorrectly configured DNS server or default gateway can prevent a computer from contacting the network.

Putting it all together
These tools are most beneficial when used together. Do you have a computer that cannot connect to the network? Use IPCONFIG with the /all switch to see if you have an IP address and your default gateway. Then use the PING command to see if you can reach your default gateway. If you cannot reach your default gateway, then use the TRACERT command to track down the communication break.

There are many more commands than these, but using these three tools will allow you to perform basic troubleshooting on your IP-enabled network.

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