Ping and ICMP

First of all what is ICMP?

The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be reached. ICMP can also be used to relay query messages. It is assigned protocol number


ICMP differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications (with the exception of some diagnostic tools like ping and traceroute).

You will come across following ICMP replies when you ping a particular IP.

When a PING is successful you get a reply back from the host showing its IP address and other information such as the time it took for the reply to go through.

TTL Expired in Transit
The TTL value determines the maximum amount of time an IP packet may live in the network without reaching its destination. It is effectively a bound on the number of routers an IP packet may pass through before being discarded. This message indicates that the TTL expired in transit. Number of required hops exceeds TTL. Increase TTL by using the ping -i switch.

Destination Host Unreachable
The host that you are trying to ping is down or is not operating on the network. A local or remote route does not exist for destination host. Modify the local route table or notify the router administrator.

Request Timed Out
The ping command timed out because there was no reply from the host. No Echo Reply messages were received due to network traffic, failure of the ARP request packet filtering, or router error. Increase the wait time using the ping -w switch.

Unknown Host
The IP Address or the Host Name does not exist in the network or the destination host name cannot be resolved. Verify name and availability of DNS servers.


This is just to explain different kind of ICMP replies I came across while reading about networking ping and tracert commands..