ARP (Address Resolution Protocol)
The Address Resolution Protocol (ARP) is a network protocol used to map a network address (such as an Internet Protocol, or IP, address) to a physical address (such as a Media Access Control, or MAC, address) in a local network. ARP is used by network devices to communicate with each other and determine where to send data packets.
In this article, we will discuss ARP in detail, including its purpose, how it works, and some common ARP-related issues.
Purpose of ARP
ARP serves two main purposes in a network:
- Resolving IP addresses to MAC addresses: When a device needs to send data to another device on the same local network, it needs to know the MAC address of the target device. However, the device typically only knows the IP address of the target device. ARP allows the device to send an ARP request to the network, asking for the MAC address that corresponds to the IP address.
- Caching ARP information: Once a device has obtained the MAC address for a specific IP address, it can cache this information for future use. This caching allows the device to avoid having to perform an ARP request every time it needs to send data to the same device.
How ARP works
When a device on a network needs to communicate with another device, it first checks its ARP cache to see if it already has the MAC address for the target IP address. If the MAC address is not in the cache, the device sends an ARP request to the network.
The ARP request is a broadcast message that is sent to all devices on the local network. The message contains the IP address of the target device and the MAC address of the device sending the request.
When a device receives an ARP request, it checks to see if the target IP address matches its own IP address. If it does, the device sends an ARP reply back to the requesting device. The ARP reply contains the MAC address of the device sending the reply.
Once the requesting device receives the ARP reply, it adds the MAC address to its ARP cache. The device can now use the MAC address to send data packets to the target device.
ARP cache
As mentioned earlier, devices on a network can cache ARP information to avoid having to perform an ARP request every time they need to send data to another device. The ARP cache is a table that stores the IP addresses and corresponding MAC addresses of devices on the network.
When a device sends a data packet to another device on the network, it first checks its ARP cache to see if it already has the MAC address for the target IP address. If the MAC address is in the cache, the device uses it to send the data packet directly to the target device. If the MAC address is not in the cache, the device must perform an ARP request to obtain the MAC address.
ARP cache entries have a limited lifetime, typically ranging from a few minutes to a few hours. After the lifetime of an entry has expired, the device will need to perform another ARP request to obtain the MAC address.
ARP spoofing
ARP spoofing is a technique used by attackers to intercept network traffic between two devices. The attacker sends fake ARP messages to the devices on the network, causing them to associate the attacker's MAC address with the IP address of the legitimate device.
Once the attacker has successfully spoofed the ARP cache of a device, they can intercept and modify any data packets sent between the two devices. This technique is commonly used in man-in-the-middle attacks.
To protect against ARP spoofing, network administrators can implement various security measures, such as using static ARP entries or implementing ARP spoofing detection software.
ARP and IPv6
ARP was developed for use with IPv4 addresses, which are 32-bit addresses that are represented as four decimal numbers separated by periods. However, IPv6 uses a different addressing scheme that is not compatible with ARP.
Instead of using ARP, IPv6 uses a different protocol called the Neighbor Discovery Protocol (NDP) to map IP addresses to MAC addresses. NDP uses a process similar to ARP, but with different message types and formats.
NDP provides additional functionality beyond what ARP can provide, such as the ability to discover routers and perform duplicate address detection.
ARP and network performance
ARP can have an impact on network performance, particularly in large networks with many devices. This is because each ARP request is a broadcast message that is sent to all devices on the local network. In a large network, this can result in a significant amount of network traffic.
To reduce the impact of ARP on network performance, network administrators can implement various techniques, such as using static ARP entries or implementing ARP caching.
Conclusion
The Address Resolution Protocol (ARP) is a network protocol used to map a network address to a physical address in a local network. ARP serves two main purposes: resolving IP addresses to MAC addresses and caching ARP information to avoid having to perform an ARP request every time data needs to be sent to the same device.
ARP cache entries have a limited lifetime, and devices can use static ARP entries or implement ARP spoofing detection software to protect against ARP spoofing attacks. In IPv6 networks, the Neighbor Discovery Protocol (NDP) is used instead of ARP to map IP addresses to MAC addresses.
ARP can have an impact on network performance, particularly in large networks, and network administrators can implement various techniques to reduce this impact.