In the previous installment, you learned some of the basics of IP addressing. There is another tidbit of information that might come in handy when you are tasked with troubleshooting connectivity on a TCP/IP network. DNS (Domain Naming Service) is a server/protocol that maintains a list of specific computer names (Fully Qualified Domain Names, FQDNs) and their corresponding IP addresses. This allows us wacky humans to attempt communication by name rather than IP address.

Now that you have an idea of how TCP/IP works, here is a generic way of testing just about every type of connectivity on your network.

1. Open a command window, and type IPCONFIG /ALL
2. Locate your current IP address.
3. If the IP address is in the range of 169.254.xxx.xxx, you may have a DHCP problem: if DHCP is on the network, then you’re not getting there, which could mean a problem with your computer, or a problem with the DHCP server.
4. Try IPCONFIG /RELEASE followed by IPCONFIG /RENEW. If your computer is configured to obtain an IP address automatically, the system will again look for a DHCP server. If the DHCP server is not, you have a problem. IF the DHCP server is found, step 1 should reveal a completely different IP address.
5. Write down the default gateway that is assigned to your computer. Also, write down the primary and secondary DNS server addresses.
6. If there is no default gateway, then either DHCP has not been configured to give you one, or you haven’t been manually configured to have one. If your intention is to access the Internet or another network, you must have a default gateway address.
7. If you have no DNS server addresses, again DHCP has not been configured to give you one, or you haven’t been configured to have one. DNS is required to access the Internet.
8. Type PING followed by the address for the default gateway.
9. If you receive a response, then local networking communications are functioning properly.
10. Type PING followed by the internet name for a Web site, such as www.lockergnome.com. The system will attempt to determine the IP address associated with that FQDN.
11. IF it is successful, the system will display the IP address for that name, and proceed to contact it. This proves that the connection between your gateway and the ISP and the destination is intact.
12. If DNS is down or not functional, the name will not resolve, and you will receive an unknown host response on the screen.
13. PING the primary and/or secondary DNS server IP address. The physical location of the server may or may not be on your local LAN. The idea is to PING the DNS server that is on the other side of the gateway.
14. If this PING is successful, you have established that the connection between your gateway and the ISP is intact, but that something is wrong between the ISP and the destination. Call the ISP to troubleshoot from there. The problem is definitely not with you.
15. If this PING is unsuccessful, you can assume that the connection between your gateway and the ISP is not intact. Again, call the ISP to troubleshoot. The problem may be ISP-based, such as the DNS server is down, or it may be that the physical router connection between the gateway and the ISP is having problems.