ICMP

The Internet Control Message Protocol (ICMP) is used for network diagnostics and error reporting. Tools like ping and traceroute rely on ICMP messages to test connectivity.

Common message types include Echo Request/Reply and Destination Unreachable notifications.

ICMP operates alongside IP but is considered part of the network layer. When a packet cannot reach its destination or when routers need to share information, ICMP generates short control messages. Each message has a type and code specifying the condition.

Key Uses

• Connectivity tests with Echo Request and Echo Reply (used by ping)
• Path discovery using Time Exceeded messages (used by traceroute)
• Error reporting such as Destination Unreachable or Redirect messages

Because ICMP packets can reveal network details, many administrators filter them at firewalls for security.