ARP (Address Resolution Protocol) is a protocol used in networking to map an IP address to a MAC address.
When a device wants to send data to another device on the same network, it will know the IP address of the destination device, but it needs to know the MAC address of the recipient so that it can send the data to the correct interface.
The sending device uses ARP to send a broadcast message to all other devices on the network, asking for the MAC address for the device with the corresponding IP address.
When the recipient device receives the broadcast message, it sends back a reply message with its MAC address, which the sender can then use to send the data.
ARP is a vital part of network communications, as it enables devices to communicate with each other and exchange data. Without ARP, devices would not be able to determine the MAC addresses of other devices on the network, and communication would not be possible.
Gratuitous ARP
A gratuitous ARP (Address Resolution Protocol) is an ARP message whereby a device on a network broadcasts its own MAC address and IP address to all other devices on the network.
Gratuitous ARPs can serve a few different purposes, including:
- Updating ARP caches: When a device changes its IP address or MAC address, it can send a gratuitous ARP to update the ARP caches of other devices on the network. This helps ensure that other devices have the most up-to-date information about the device’s address.
- Detecting IP address conflicts: If two devices on a network have the same IP address, this can cause communication issues. By sending a gratuitous ARP, a device can detect if there is an IP address conflict, as it will not receive a response from the other device with the same IP address.
- Network redundancy: Some network devices, such as routers or load balancers, may use gratuitous ARP messages to announce their presence on the network and advertise their capabilities, such as providing redundancy or failover support.
Gratuitous ARPs are not required for normal network communication, but they can be useful for ensuring network reliability and troubleshooting network issues.
ARP Cache
An ARP cache is a table or database on a device that contains information about the MAC addresses of other devices on the same network, along with their corresponding IP address.
When a device needs to send data to another device on the network, it first checks its ARP cache to see if it already has the MAC address of the recipient device. If the address is found in the cache, the device can send data directly to that device without having to perform an ARP broadcast to discover the physical address.
The ARP cache is populated via ARP requests and gratuitous ARP broadcasts. When a device receives ARP data, it stores the MAC and IP address pair in its ARP cache, along with a time-to-live (TTL) value that indicates how long the entry should remain in the cache.
The ARP cache can improve network performance by reducing the number of ARP broadcasts required for communication between devices. However, if a device’s ARP cache becomes stale or inaccurate, it can cause communication issues on the network. For this reason, ARP caches are typically periodically refreshed or cleared to ensure that they contain accurate information about the devices on the network.
Checking your ARP cache
On a Windows device, you can check your ARP cache by opening a CMD prompt and typing arp -a
On a Linux device, the ARP cache is located at /proc/net/arp. and can be viewed via the cat command or other similar utiltity.
