Internet Control Message Protocol (ICMP)
Data sent to a remote computer often travels through one or more routers; these routers can encounter a number of problems in sending the message to its ultimate destination. Routers use Internet Control Message Protocol (ICMP) messages to notify the source IP of these problems. ICMP is also used for other diagnosis and troubleshooting functions.
The most common ICMP messages are listed here. Quite a few other conditions generate ICMP messages but their frequency of occurrence is quite low.
Echo Request and Echo Reply— ICMP is often used during testing. A technician who uses the ping command to check connectivity with another host is using ICMP. ping sends a datagram to an IP address and requests the destination ...
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