ICMP types

Here are some of the common ICMP types used to discover a host on a network. You can get complete descriptions of ICMP types from RFC 792.

Message Type: 0 - Echo Reply
Message Type: 3 - Destination Unreachable
Message Type: 4 - Source Quench
Message Type: 5 - Redirect
Message Type: 8 - Echo
Message Type: 11 - Time Exceeded
Message Type: 12 - Parameter Problem
Message Type: 13 - Timestamp
Message Type: 14 - Timestamp Reply
Message Type: 15 - Information Request
Message Type: 16 - Information Reply

While ICMP might be useful for a network administrator to know what is going on it is also useful for a hacker to know what is going on. Denying ICMP requests from outside your network is one way to help deter hackers.