Testing Network Connectivity with the PING Command

Perhaps no command line utility is more familiar to users than PING. On a TCP/IP network, the PING utility allows you to determine whether another system is reachable, while at the same time providing basic diagnostic information such as whether any packets are being lost en route. When troubleshooting a network connection, PING is almost always the first tool that any users will unleash to gather basic information. When issued with the –t command, PING will continuously send requests to a host until manually stopped with the CTRL+C command.

Checking TCP/IP Network Settings with IPCONFIG

If you’re connected to a TCP/IP network, the IPCONFIG utility is one that you need to be familiar with. Most users are familiar with the tool from using it to renew an IP address allocated by a DHCP server via the /renew switch. However, IPCONFIG provides a wealth of useful information about your system’s TCP/IP settings, especially when the /all switch is issued. This command will display information about the state of your connection, whether DHCP is being used, the IP address, subnet mask, and default gateway configured on your system, and more.

Finding System MAC Addresses with the ARP Command

If your computer has an Ethernet or Wi-Fi network adapter card installed and is connected to a TCP/IP network, the Address Resolution Protocol is used to map IP addresses of systems you are communicating with on the local network to their network card’s physical MAC address. The ARP command allows you to view the MAC addresses of systems with which your system is communicating, such as your local router interface. This information can be useful when troubleshooting connection problems, or when a duplicate IP address is configured on your network.

To view the contents of the ARP cache on your system, issue the ARP –a command. If your ARP cache contains no entries, ping another computer on your network, and issue the command again.

Getting Familiar with TCP/IP Port Numbers

Certainly you don’t need to know anything about port numbers if you never plan to allow external users to access your network, or if you don’t plan to control the types of Internet services that your internal users can use. However, if you do plan to make use of either feature, you’ll need to know something about port numbers.

Different types of applications use different port numbers to communicate. Port numbers come in two flavors, namely TCP and UDP. Transmission Control Protocol (TCP) is a reliable protocol used by some applications (such as Web, FTP, and Email servers), while User Datagram Protocol (UDP) is a faster (but unreliable) protocol used by services like DNS. You don’t get to choose which is used – the specifications for different services define which protocol is used which individual applications.

A total of 65536 TCP/UDP port numbers exist. Certainly no one could remember all of them, but some of them are much more common than others. For example, the list below outlines some of the port numbers used by common services:

HTTP (Web servers) – TCP 80
FTP (FTP servers) – TCP 21
SMTP (Email servers) – TCP 25
POP3 (Email servers) – TCP 110
DNS (Name resolution) – UDP 53

This is far from a comprehensive list, but gives you the idea. So, if you plan on having your own internal FTP server that should be accessible from the Internet, port forwarding would need to be enabled on your router for TCP ports 20 and 21. If you’re using ICF, a definition for FTP already exists which you can simply check off to accomplish the same task. For a complete and very comprehensive list of port numbers, see http://www.iana.org/assignments/port-numbers.

Windows XP Network Diagnostic Tools

If you’ve ever tried your hand at configuring a home network, chances as good that you’ve run into communications problems at some point. All Windows versions include a variety of network diagnostic tools, although you’ll find more in Windows XP than Windows 98. Regardless of your operating system version, the two basic tools that you’ll want to be familiar with include both the ipconfig and ping command line utilities.

The basic purpose of ipconfig is to allow you to view basic TCP/IP information about your system including its IP address, subnet mask, and default gateway. Conveniently, this tool will also let you know if your network cable is unplugged. Typing ipconfig at the prompt provides basic information, but typing ipconfig /all provides variety of additional data, including how your system acquired its address, for example statically or dynamically. If your system has what appears to be an address starting with 169.254, this likely means that a DHCP server wasn’t available – to try to acquire an address again, type ipconfig /renew and press Enter.

In the world of networking, ping is the most basic diagnostic utility to test communications. If you cannot connect to a system, try to ping its IP address using the format ping You can also use the name of the server, for example ping www.pcanswers.co.uk. If you’re trying to fix your own system and want to ping continuously for testing purposes, use the –t option, for example ping –t Finally, if you want to try to obtain the name of the computer for which you already know the IP address, type ping –a, and the name of the system will usually be returned.

Windows Networking Utilities

Once TCP/IP is installed and configured on the computers on your network, a variety of helpful and interesting diagnostic and troubleshooting utilities become available to you. Most of these utilities are meant to be run from the command line, so make sure that you have that command prompt icon nice and handy on your desktop. The list below outlines some of the common utilities that you’ll want to be familiar with, along with their primary functions, and examples of how they are used.

PING – The most basic and essential of the TCP/IP utilities, the PING command is used to test basic connectivity on a TCP/IP network. When you ping another host on your network, the machine from which the command is used sends out an “echo request” message, and then determines success by whether it receives back an “echo reply”. When echo reply messages are received, it means that the two computers are capable of communicating via TCP/IP. PING is the first utility that should always be used when attempting to troubleshoot a connectivity issue on a TCP/IP network. The ping command can be used with IP addresses or FQDNs. For example, to ping the PC Answers web server, you would type ping www.pcanwers.co.uk, press Enter. If you receive 4 echo reply messages, you’re likely up and running correctly.

IPCONFIG – The IPCONFIG command represents the easiest way to gather TCP/IP configuration information for your computer from the command line. Instead of accessing your network properties through the Windows interface, simply type ipconfig at the command prompt and press Enter. You will be provided with information on the IP address, subnet mask, and default gateway values configured on your PC. For more comprehensive information (including the IP addresses of DNS servers), type ipconfig /all and press Enter. If you’re running Windows 2000 or XP, try using the ipconfig /displaydns command to view the FQDNs that your system has resolved to IP addresses.

TRACERT – One exceptionally interesting TCP/IP command used to troubleshoot network connectivity issues is TRACERT. The purpose of the TRACERT command is to trace the route that a packet takes between a source and destination host. For example, when you cannot ping a host, it does not necessarily mean that the host is unavailable. Instead, it might mean that a problem exists somewhere on the path between the two hosts. When the TRACERT command is issued with an IP address or FQDN, it will report back with information on the entire path taken (namely the routers crossed) in trying to reach the destination network. For example, try typing tracert www.yahoo.com and press Enter. This command will display all of the routers crossed between your PC and the Yahoo web server, as shown above – probably more than you would have thought!

NETSTAT – The NETSTAT command is useful when attempting to determine the status of connections between your computer and other computers on your network or the Internet. From the command line, type netstat and press Enter. The results will show you both the systems that this computer is connected to, along with the status of the connections.
Traceroute.bmp: Use the TRACERT utility to determine the path that a packet takes between a course and destination host.