Understanding ICMP (Internet Control Message Protocol)
One helpful analogy for ICMP is sending a parcel in the post and using a tracking number to understand where it is up to along the way. As a courier picks up the package from a depot they scan it, and that information about what has occured, and where, makes its way back to you via that tracking number to let you know that latest updates.
ICMP has developed with IP (Internet Protocol) over time, and evolves with it. As there are drastic changes between IPv4 and IPv6, we see ICMPv6 protocol emerge, while leaving IPv4 ICMP in-use for IPv4. This FAQ will simply focus on IPv4 ICMP, and how it helps service providers, networks (both the routing equipment, and the operators), as well as end users, with troubleshooting and understanding their networks.
Main Types of Messages
Starting with a a quick understanding of the main ICMP message types is essential, and makes reading and understanding pings and traceroutes simple and straight-forward.
|0||PING Echo Reply||This is the ICMP message type used to send the reply to a ping request.|
|8||PING Echo Request||This is the ICMP message type of a ping request.|
|3||Destination Unreachable||There are 16 codes defined within this type of message that are used by routers to inform your computer about how exactly the destination is unreachable.|
|11||Time Exceeded||This informs your computer that the detination was not reached within the number of hops requested.|