ICMP (Internet Control Message Protocol)

ICMP (Internet Control Message Protocol) is a Layer 3 Protocol used in IP networks. It is sent as encapsulated in IP packet. The general role of ICMP is generate error packets about the network devices and it is used in troubleshooting activities. As a summary, DHCP supports IP routing, diagnostics and error reporting.

ICMP is generally used by network engineers for troubleshooting activities. The main two troubleshooting commands for network engineers , “ping” and “traceroute”, works with ICMP. In every passed node, TTL value in the IP packet decrease one. And when it is 0, ICMP message is sent to the sender, about the reachability of the destination.

ICMP does not guaranteed the delivery. A hundred percent security is not required already.

ICMP Message and ICMP Header Format

ICMP Message is sent as encapsulated in IP Packet.


In the ICMP Message, firstly an ICMP Header is added to the front.This ICMP Message, consist of the below areas:

• Type
• Code
• Checksum
• Unused


ICMP Messages can be divided into two category accourding to their goals. These Message Categories are:

• Error Messages
• Informational / Query Messages

ICMP Type And Code Fields

ICMP Header has three main areas as we mentione above: Type, Code and Checksome. Here;

• “Type” is the format of DHCP Message.
• “Code” is a detailed description of DHCP Message.

What are these Type and Code values? Let’s see firtly Type values and their meanings. And then we will talk about the Code values.

The Type values and teir meanings are given below:

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Like ICMPv4, ICMPv6 also has the Code values. You can also check the code values of this different Types below:




Lesson tags: icmp
Back to: HCNA > ICMP and ARP