What is Gratuitous ARP

Gratuitous ARP is a special ARP (Address Resolution Protocol) reply that is not a response to an ARP request. A Gratuitous ARP reply is a reply to without a ARP request. No reply is expected for a Gratuitous ARP . A Gratuitous ARP packet has the following characteristics.

Gratuitous ARP packets are generated by network devices for some of the reasons listed below.

For example, let us consider below topology.

Gratuitus ARP 01

Let us assume that for some technical requirement, the IPv4 address of the top left side computer is changed to from

Gratuitus ARP 02

The change in IPv4 address must be informed to all other devices in the Local Area Network (LAN), because there may be MAC address to incorrect previous IPv4 address mappings inside the ARP table of other network devices. Gratuitous ARP is mainly used by a TCP/IP device to inform other devices in the Local Area Network (LAN), any change in its MAC address or IPv4 address.

Since the destination MAC address is the broadcast MAC address, the switch will flood the Gratuitous ARP packet to all its connected ports.

Following Wireshark capture screenshot shows a Gratuitous ARP packet.

Gratuitus ARP


Related Tutorials
Introduction to TCP/IP, Features of TCP/IP, TCP/IP History, What is RFC (Request for Comments)
Seven Layers of OSI Model and functions of seven layers of OSI model
Five layered TCP/IP model
Network Layer (Layer 3)
TCP/IP Data Encapsulation and Decapsulation
Datalink Layer (Layer 2)
Ethernet Frame Format
What is MAC address or Layer 2 address or physical address
IPv4 Protocol, IPv4 header and fields of IPv4 header
IPv4 addresses, IPv4 Address Classes, IPv4 Address Classifications
What is Subnet Mask
What is network address
Limited broadcast address
Directed broadcast address
What is limited broadcast in IPv4 and how limited broadcast works
What is directed broadcast in IPv4 and how directed broadcast works
What is default gateway
What are private IP addresses - RFC 1918 private addresses
APIPA Addresses (Automatic Private IP Addresses)
What is loopback address in IPv4
What is localhost
Class A networks and Class A IP addresses
Class B networks and Class B IP addresses
Class C networks and Class C IP addresses
Class D multicast IP addresses
Subnetting - Part 1
Subnetting - Part 2
Subnetting - Part 3
Subnetting - Part 4
Subnetting - Part 5
Subnetting - Part 6
Variable Length Subnet Masking, VLSM, IP V4 Subnetting, subnetting tutorials, IP study guides, IP documentation, IP tutorials
Supernetting, IP Supernetting, IP Supernetting tutorial, How to Supernet, Supernetting Guide, Supernetting Concepts
How to find out the Network Address and Broadcast Address of a subnetted IPv4 address
Address Resolution Protocol Tutorial, How ARP work, ARP Message Format