ARP (Address Resolution Protocol) and GARP (Gratuitous ARP) are both network protocols used in Ethernet networks to resolve IP addresses to MAC (Media Access Control) addresses. However, they serve different purposes:

Address Resolution Protocol (ARP):

ARP is used to resolve an IP address to its corresponding MAC address within a local network. When a device wants to communicate with another device on the same network, it needs to know the MAC address of the destination device. The ARP protocol allows a device to broadcast an ARP request, asking “Who has this IP address?” The device with the corresponding IP address responds with its MAC address, allowing the requesting device to create an ARP cache entry, mapping the IP address to the MAC address. This ARP cache entry is then used for subsequent communication with the device.

Gratuitous ARP (GARP):

GARP, on the other hand, is a variation of ARP used for different purposes. It is an unsolicited ARP message broadcasted by a device on the network to update or announce its IP-to-MAC address mapping. Unlike a regular ARP request, GARP does not include a question (“Who has this IP address?”). Instead, it contains a statement (“I have this IP address, and my MAC address is this”). GARP is typically used in the following scenarios:

  • IP Address Conflict Detection: A device can send a GARP message to verify if the IP address it wants to use is already in use by another device on the network. If it receives a response to its GARP message, it knows that there is an IP address conflict and can take appropriate action to resolve it.
  • Network Redundancy and Failover: In redundant network setups, such as High Availability (HA) configurations, a device can send GARP messages to announce its presence and update its MAC address mapping. This ensures that other devices on the network are aware of the active device and can update their routing or forwarding tables accordingly.
  • Update ARP Cache in Nearby Devices: By broadcasting a GARP message, a device can proactively update the ARP caches of other devices on the network, ensuring they have the most up-to-date IP-to-MAC address mapping. This can be useful in certain network management scenarios.

In summary, ARP is used for resolving IP addresses to MAC addresses in response to specific requests, while GARP is an unsolicited message used for IP address conflict detection, network redundancy, and updating ARP caches in nearby devices.