What is icmp

Last updated: April 1, 2026

Quick Answer: ICMP (Internet Control Message Protocol) is a network protocol that sends error messages and diagnostic information over the internet, most commonly used by the ping utility to test network connectivity and diagnose problems.

Key Facts

ICMP is defined in RFC 792 and operates at Layer 3 (Network Layer) of the OSI model as part of the Internet Protocol Suite
The ping utility uses ICMP Echo Request and Echo Reply messages to test whether a host is reachable and measure response time
ICMP is used by the traceroute utility to map network paths by sending messages with incrementally increasing TTL values
ICMP can report various network errors including destination unreachable, time exceeded, and parameter problems
ICMP messages are identified by type numbers, with types 0-42 and beyond representing different message categories and functions

What is ICMP?

The Internet Control Message Protocol (ICMP) is a core network protocol used to send error messages and diagnostic information across IP networks. Unlike TCP and UDP which carry user data, ICMP is designed to provide feedback about network conditions and issues. ICMP is essential for network troubleshooting and communication between network devices about delivery problems and network status.

How ICMP Works

ICMP operates by encapsulating messages within IP packets. When a network problem occurs—such as a host being unreachable or a router being unable to forward a packet—ICMP generates an error message that is sent back to the source. ICMP can also be used proactively to request information about network conditions. The protocol is connectionless, meaning it doesn't establish a connection before sending messages. Each ICMP message includes a type field that identifies the message category and a code field that provides additional specificity about the condition being reported.

Common ICMP Applications

Ping is the most familiar ICMP application. It sends ICMP Echo Request messages to a destination and waits for Echo Reply messages, allowing users to verify that a host is reachable and measure latency. Traceroute uses ICMP to discover the route packets take across the internet by sending messages with progressively higher Time-To-Live (TTL) values, causing intermediate routers to return error messages that reveal their addresses. Network administrators use these tools daily to diagnose connectivity problems, measure network performance, and identify routing issues.

ICMP Message Types

ICMP defines numerous message types for different purposes. Echo Request (type 8) and Echo Reply (type 0) are used by ping. Destination Unreachable (type 3) reports that a packet cannot be delivered. Time Exceeded (type 11) indicates that a packet's TTL has expired. Redirect (type 5) suggests a better route. Parameter Problem (type 12) indicates an invalid IP header. Each type has associated codes that provide more specific information about the network condition being reported.

More What Is in Daily Life

What Is a Credit ScoreA credit score is a three-digit number, typically ranging from 300 to 850, that represents your cred…
What Is CD rates make no sense based on length of time invested. Explain like I'm 5CD (Certificate of Deposit) rates often don't increase with longer lock-up times the way people expe…
What is a phdA PhD (Doctor of Philosophy) is a doctoral degree earned after completing advanced academic research…
What is a polymathA polymath is a person with deep knowledge and expertise across multiple different fields or academi…
What is aaveAAVE stands for African American Vernacular English, a dialect with distinct grammar, pronunciation,…
What is aarch64ARMv8-A (commonly called ARM64 or AArch64) is a 64-bit processor architecture developed by ARM Holdi…
What is about menTopics and discussions about men typically encompass masculinity, male identity, gender roles, men's…
What is abiturAbitur is the German academic qualification awarded upon completion of secondary education, typicall…
What is abrosexualAbrosexual is a sexual orientation identity where a person's sexual attraction changes or fluctuates…
What is abgABG is an Indonesian acronym standing for 'Anak Baru Gede,' which refers to adolescent girls or teen…
What is aaaAAA batteries are a standard cylindrical battery size measuring 10.5mm in diameter and 44.5mm in len…
What is aacAAC (Advanced Audio Codec) is a digital audio compression format that provides better sound quality …
What is aaa gameAAA games are high-budget video games developed by large studios with budgets typically exceeding $1…
What is a proxyA proxy is a server that acts as an intermediary between your device and the internet, forwarding yo…
What is ableismAbleism is discrimination and prejudice against people with disabilities based on the assumption tha…
What is absAbs, short for abdominal muscles, are the muscles in your core that flex your spine and stabilize yo…
What is abortionAbortion is a medical procedure that ends pregnancy by removing the fetus before viability. It can b…
What is accutaneAccutane (isotretinoin) is a powerful prescription medication derived from vitamin A used to treat s…
What is acetaminophenAcetaminophen, also known as paracetamol, is an over-the-counter pain reliever and fever reducer use…
What is acidAcid is a chemical substance that donates protons (hydrogen ions) to other substances, characterized…