Explain the purpose of ARP (Address Resolution Protocol).

The Address Resolution Protocol (ARP) is a communication protocol used in computer networks to map an Internet Protocol (IP) address to a physical machine address (Media Access Control address or MAC address). ARP plays a crucial role in the functioning of Ethernet networks, as well as other types of networks.

Here's a more detailed technical explanation of the purpose of ARP:

  1. Address Mapping:
    • IP Address to MAC Address Mapping: In a TCP/IP network, each device is identified by a unique IP address. However, when data is transmitted over the physical network, it needs to be addressed using the MAC address of the target device. ARP is responsible for creating and maintaining a mapping between the IP address and the MAC address of devices in the local network.
  2. Data Link Layer Interaction:
    • Operates at Data Link Layer (Layer 2): ARP operates at the data link layer of the OSI model. It is specifically designed for Ethernet and other similar technologies. At this layer, data frames are encapsulated with MAC addresses.
  3. ARP Request and Reply:
    • ARP Request: When a device wants to communicate with another device on the same network and knows the IP address but not the corresponding MAC address, it sends an ARP request broadcast packet to all devices on the local network.
    • ARP Reply: The device with the specified IP address sends back an ARP reply packet containing its MAC address to the requesting device. This reply is then used by the requesting device to build an ARP cache or table, associating the IP address with the corresponding MAC address.
  4. ARP Cache:
    • Cache for Efficiency: To avoid unnecessary ARP broadcasts for frequently used IP addresses, devices maintain an ARP cache or table. This cache stores recent mappings of IP addresses to MAC addresses, allowing devices to quickly retrieve this information without the need for additional ARP requests.
  5. Dynamic and Static ARP:
    • Dynamic ARP: The standard operation of ARP is dynamic, meaning the mappings are learned and updated as devices communicate on the network.
    • Static ARP: In some cases, static ARP entries can be manually configured to ensure specific IP-to-MAC mappings. This is less common and typically used in specific network configurations.
  6. Proxy ARP:
    • Proxy ARP: In certain network setups, a device may answer ARP requests on behalf of another device, effectively acting as a proxy. This is known as Proxy ARP and is used in scenarios where devices are not directly reachable due to network configurations.