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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. Figure 4-15 outlines the basic process.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 4-15 Sample Network, ping Command

Note that while the ping command uses ICMP, ICMP does much more. ICMP defines
many messages that devices can use to help manage and control the IP network. Chapter 20,
“DHCP and IP Networking on Hosts,” gives you more information about and examples of
ping and ICMP.

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