by Eileen Grace

Created on: August 24, 2009

Ping is one of the most basic tools for diagnosing network problems, and as such is an essential part of the toolkit for anyone involved in computer networking. It's a very simple test - determining whether there is a response from the system at a given IP address.

*History

The ping command was created in 1983 by Michael Muuss, and was named for its similarity to the sonar echo-location technique used by submarines. Originally created for Unix, ping has since been ported to virtually all operating systems.

*What it does

Ping uses ICMP (Internet Control Message Protocol) to send a special data packet containing an echo request from one system to another on a TCP/IP network. If the receiving system is active on the network, it sends back a reply which includes the original request in the body.

*Using ping

To use the ping command from the most common modern operating systems is very simple: open a command window (MS-DOS prompt in Windows 9x, command prompt in Windows NT/2000/XP/Vista, or a terminal window in OS X, Linux, BSD or other Unix system) and enter "ping" followed by the address to be tested. Either an IP address or hostname can be used, for example to ping a system at 192.168.2.53 with the hostname "bart", you would enter either "ping 192.168.2.53" or "ping bart".

In Unix, this basic command will result in a series of pings running until stopped, while under Windows four packets are sent. The results for each ping sent will be displayed on the screen of the sending system as follows:

For successful pings you'll see something like "Reply from 192.168.2.53: bytes=32 time=761ms TTL=127", while failures will report "Reply timed out", "unknown host" or "network unavailable".

The information in the successful response gives the address of the responding system, size of the packet, how long the packet took to get from the originating system to the responding system and back, and the remaining Time To Live (TTL). When the packet is first sent out it has a TTL of 255 which is reduced by 1 for every hop it makes in reaching the responding system.

Below the reply listing, there will also be statistics shown:
Ping statistics for 192.168.2.53:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 1ms, Average = 0ms

The ping command is useful for testing network conditions. You can ping an address that exists within your LAN or over the Internet. Sometimes, when your Internet or LAN connection seems to be down, pinging can tell you whether or not the problem concerns the name resolution or whether or not that computer can be reached at all.

For example, if pinging bart returned this message:
Unknown host bart

but pinging bart's IP address 192.168.2.53 was successful, you know that the computer is on the network, but for some reason the name bart is not being recognized which points to a DNS issue.

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