PING is an application that sends packets to a specified destination. Each packet that PING sends is a request for a response from the destination node. The destination node sends a response packet to the source of the request after it receives each request.

PING uses a protocol from the TCP/IP suite of protocols called the Internet Control Messaging Protocol (ICMP). The request packets are called ICMP ECHO_REQUESTs. The response packets are called ICMP echo replies. There are a number of ICMP messages that can be returned when an ECHO_REQUEST is received. Table 12.1 lists the possible ICMP response messages.

Table 12.1. ICMP Response Messages
Message Description
Network unreachable Gateway has no route to destination network ...

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