TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 3-38 ICMP: Address Mask Request and Reply
3.2.2 ICMP applications
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identifier sequence number
subnet address mask
There are two simple and widely used applications based on ICMP: Ping and
Traceroute. Ping uses the ICMP Echo and Echo Reply messages to determine
whether a host is reachable. Traceroute sends IP datagrams with low TTL values
so that they expire en route to a destination. It uses the resulting ICMP Time
Exceeded messages to determine where in the internet the datagrams expired
and pieces together a view of the route to a host. We discuss these applications
in the following sections.
Ping
Ping is the simplest of all TCP/IP applications. It sends IP datagrams to a
specified destination host and measures the round trip time to receive a
response. The word ping, which is used as a noun and a verb, is taken from the
sonar operation to locate an underwater object. It is also an abbreviation for
Packet InterNet Groper.
Generally, the first test of reachability for a host is to attempt to ping it. If you can
successfully ping a host, other applications such as Telnet or FTP should be able
to reach that host. However with the advent of security measures on the Internet,
particularly firewalls (see 22.3, “Firewalls” on page 794), which control access to
networks by application protocol or port number, or both, this is no longer
necessarily true. The ICMP protocol can be restricted on the firewall and
therefore the host is unable to be successfully pinged.
The syntax that is used in different implementations of ping varies from platform
to platform. A common format for using the ping command is:
ping host
Where host is the destination, either a symbolic name or an IP address.
Most platforms allow you to specify the following values:
Size The size of the data portion of the packet.
Packets The number of packets to send.
Count The number of echo requests to send.
Chapter 3. Internetworking protocols 117