ICMP Packet Header

239

Type

Code

Checksum

ICMP Information

Figure C 2 Basic ICMP header

An explanation of the fields in a basic ICMP header is provided in Table C 2.

Table C 2 ICMP Packet Header Fields

Field

Explanation

Type

This part is 8 bits long and shows the type of ICMP packet.

Code

This part is also 8 bits long and shows the sub type or code number used for the packet.

Checksum

This part is 16 bits long and is used to detect any errors in the ICMP packet.

The ICMP information part is variable depending upon the value of the type field.

For example, the ping command uses ICMP ECHO REQUEST type packet. This

packet header is shown in Figure C 3.

Type

Code

Checksum

Identifier

Sequence Number

Figure C 3 ICMP packet used in ping command.

For a complete list of ICMP packet types, refer to RFC 792.