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ICMP Packet Header
Internet Control Message Protocol
ICMP message types
possibly a message subtype
An ICMP packet is a network layer, IP control and status message. Its header contains the ICMP message type. An ICMP message type indicates whether the packet is a command, a response to a command, status information, or an error condition. ICMP messages do not contain source and destination ports. They are not sent between programs. They are sent between the computers.