TCP/IP Troubleshooting Tools

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[edit section] TCP/IP Troubleshooting Tools

TCP/IP is a wonderful protocol suite; it comprises almost all of the functionality and the core services that make possible for the Internet and its applications. However, with great power come many problems; so, knowledge of TCP/IP troubleshooting will be necessary in your networking-related career and especially in your ability to pass the Network+ exam. In fact, you will probably encounter around four or five questions on TCP/IP troubleshooting alone on the Network+ exam.

Your knowledge of TCP/IP troubleshooting depends on your command of TCP/IP tool usage. Many of the following tools are used in typical troubleshooting operations, and almost all of them can be used to either identify or eliminate a potential problem in a troubleshooting situation. We will go over general network troubleshooting in more detail in later articles.

To give an example of the way TCP/IP troubleshooting tools can help, consider the common tool “ping.” Ping operates over the ICMP protocol (using ICMP Echo Request and Echo response) to attempt to contact a host given some kind of unique identifier (hostname, domain, IP, etc.). If it is successful, it will return a reply from that IP address; if it is unsuccessful, it will inform you that the “destination could not be reached.” Why is this useful? Suppose you are trying to determine why you are unable to access the company’s remote email server through “Outlook.” The problem could stem from a number of issues, including

• The computer is not properly configured for the internal network (media problems, TCP/IP configuration issues, router is down, etc.)

• The Internet connection is down

• There is an issue in connecting to the email server at the Application or Connection layers (maybe he is connecting to the wrong port, NAT issues, email server is rejecting connections)

• The remote host is down or is unavailable in general (IP connection issues)

Obviously, these are only four of many possibilities, but they are four possibilities that can be further investigated through Ping. For example, to ping your router would indicate to you if your router or network connection is down, or if the problem lies at the remote host level. This kind of “step-by-step, process of elimination” process is how most troubleshooting takes place. Some other tools include:

• Tracert: Traces the connection path to a remote host, step-by-step. Allows you to see where the connection “is lost” – for example, sometimes a connection to a remote host may stop at an ISP router, at which point you can determine that the issue is a WAN problem.

• Netstat: Depending on the parameters, gives varying degrees of information about TCP/IP connections and protocols. Examples include information such as all connections and listening ports, routing table etc

• Ipconfig: Displays IP configuration information with the switch (/all), which includes IP address, subnet mask, and default gateway. Ipconfig can also be used to force a DHCP release or renew operation, using the switches /release and /renew
• ifconfig: Displays IP configuration information in a UNIX environment.  ifconfig interface_name can also be used to display the IP configuration information for a specific interface

• Winipcfg - the GUI-based Windows 9x-era ipconfig tool

• ARP – Returns the MAC address that maps from a given IP address

• RARP – Returns the IP address that maps to a given MAC address

• Nslookup – Used to troubleshoot DNS issues; can be used to find an IP given a DNS name

There are of course other TCP/IP tools, but they will be covered in other sections (i.e. NetBios). The most important things to remember about the above TCP/IP tools are not the details of their functionality, but rather, remember the troubleshooting operations that they are associated with. For example, if you read about some sort of DNS issue, you should immediately think Nslookup before considering other tools.

[edit section] Quick Practice

1. A user reports that he is unable to connect to your company’s network. The user is running Windows 98. Which of the following commands would be most appropriate to run?

a. ipconfig

b. winipcfg

c. winipconf

d. ipconf

2. Which of the following commands can be used to renew a DHCP leased address?

a. Dhcp /renew

b. Dhcp /lease?extend

c. Ipconfig /renew

d. Ipconfig /extend

3. Which utility shows active connections on a host?

a. Netstat

b. Telnet

c. Tracert

d. ICMP

4. Which of the following uses ICMP echo requests to determine if a remote host is available?

a. Ping

b. Telnet

c. Arp

d. ICMP

[edit section] Answers

1. Winipcfg is the equivalent of ipconfig for Windows 9x machines. The answer is B

2. Ipconfig with the switch “/renew” can renew a DHCP lease; the answer is C

3. Netstat shows active connections on a host. The answer is A

4. Ping uses ICMP echo requests to check if a host is reachable. The answer is A