ICMP Echo and the ping Command
After you have implemented a TCP/IP internetwork, you need a way to test basic IP connectivity without relying on any applications to be working. The primary tool for testing basic network connectivity is the ping command.
Ping (Packet Internet Groper) uses the Internet Control Message Protocol (ICMP), sending a message called an ICMP echo request to another IP address. The computer with that IP address should reply with an ICMP echo reply. If that works, you successfully have tested the IP network. In other words, you know that the network can deliver a packet from one host to the other and back. ICMP does not rely on any application, so it really just tests basic IP connectivity—Layers 1, 2, and 3 of the OSI model. ...
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