ICMP Echo and the ping Command

IP needs to have a way to test basic IP connectivity, without relying on any applications to be working. Hannah, being a great network troubleshooter (in spite of being my 2-year-old daughter), can test basic network connectivity using 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. 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 5-10 outlines the basic process.

Figure 5-10 Sample Network, ping Command

Hannah n

Eth

IP

ICMP Echo Request

Eth

Eth

IP

ICMP Echo Reply

Eth

ICMP contains many features, which are discussed in detail in Chapter 13, "Basic Router Configuration and Operation."

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