ICMP is a network-layer control protocol that supports IP by communicating host reachability and diagnostic information. The ping utility uses ICMP echo requests and echo replies to test whether two hosts can reach each other; both messages are unicast. section 6.5 section 6.6
A ping also provides practical timing and result indicators:
- Round-trip time (RTT) measures how long a message takes to travel to the destination and return. section 6.5
- Cisco IOS sends five echo requests for one ping command; exclamation marks indicate replies, while periods indicate requests without replies. section 6.5
PC1# ping 10.0.0.12 #1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.0.12, timeout is 2 seconds: !!!!! #2 Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms #2 #1 Sends five ICMP echo requests (pings) to the specified IP address #2 Each exclamation mark indicates a successful ping.
A representative Cisco IOS ping shows five ICMP echoes, a 100-percent success rate, and minimum, average, and maximum RTT. section 6.5
