What is ARP and how does it work?

ARP, which stands for Address Resolution Protocol, is a communication protocol used in computer networks to map an IP (Internet Protocol) address to a physical (MAC) address. This process is crucial for devices on a network to communicate with each other. ARP operates at the data link layer (Layer 2) of the OSI (Open Systems Interconnection) model.

Here's a technical explanation of how ARP works:

  1. Need for ARP:
    • In a network, devices communicate using IP addresses at the network layer (Layer 3). However, at the data link layer (Layer 2), devices are identified by physical addresses, known as MAC (Media Access Control) addresses.
    • ARP is needed when a device wants to communicate with another device on the same local network, but it only knows the IP address of the target device and not its MAC address.
  2. ARP Table (Cache):
    • Each device on a network maintains an ARP table (also called ARP cache) that keeps a mapping of IP addresses to MAC addresses. This table is used to speed up future ARP requests by caching previous mappings.
  3. ARP Request:
    • When a device wants to send data to another device on the local network and knows the target's IP address but not its MAC address, it broadcasts an ARP request packet to the entire network.
    • The ARP request contains the sender's IP and MAC addresses, the target IP address, and a placeholder for the target MAC address.
  4. ARP Reply:
    • The device with the matching IP address in the ARP request responds with an ARP reply packet. This reply contains its IP and MAC addresses.
    • The sender updates its ARP table with the received information, associating the IP address with the MAC address.
  5. ARP Caching:
    • The sender caches the ARP reply in its ARP table, reducing the need for ARP requests in the future. Entries in the ARP table have a time-to-live (TTL) value, after which they expire and need to be refreshed.
  6. ARP Spoofing:
    • ARP is susceptible to security issues like ARP spoofing, where malicious actors send fake ARP replies to redirect traffic. To mitigate this, some networks use security measures like ARP inspection.