ICMP (Internet Control Message Protocol)

ICMP (Internet Control Message Protocol) is a network layer protocol that reports errors and provides information related to IP packet processing. Network devices use this protocol for certain purposes. This helps in sending error messages. For example, an indication for a requested service that is not available or that a host isn’t reachable.

Network tools such as ping or traceroute commonly use Internet Control Message Protocol. Consider the following example that illustrates how ping can be used to test the reachability of a host:

ICMP (Internet Control Message Protocol)

Host A wants to test whether it can reach Server over the network. Host A will start the ping utility that will send ICMP Echo Request packets to Server. If Server is reachable, it will respond with ICMP Echo Reply packets. If Host A receives no response from Server, there might be a problem on the network.

ICMP messages are encapsulated in IP datagrams. This means that they don’t use higher level protocols (such as TCP or UDP) for transmission.

ICMP (Internet Control Message Protocol)

Host A sends a packet to Host B. Because the Host B is down, the router will send an ICMP Destination host unreachable message to Host A, informing it that the destination host is unreachable, e.g.:

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