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Explain the role of ARP (Address Resolution Protocol) in IP networking.

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Address Resolution Protocol (ARP) plays a crucial role in IP networking by facilitating the mapping of an IP address to a corresponding hardware (MAC) address in a local network. The primary purpose of ARP is to resolve the layer 2 (Data Link layer) address when only the layer 3 (Network layer) address is known.

  1. Address Mapping:
    • In an IP network, each device on a local network is identified by a unique IP address (Layer 3 address) and a corresponding MAC address (Layer 2 address).
    • IP addresses are used for logical routing, while MAC addresses are used for communication within the local network.
  2. Need for ARP:
    • When a device in a local network wants to communicate with another device, it needs to know the MAC address of the target device.
    • If the target device's IP address is known but its MAC address is not, ARP is used to resolve the MAC address.
  3. ARP Request:
    • When a device wants to discover the MAC address associated with a specific IP address, it broadcasts an ARP request packet to the entire local network.
    • The ARP request contains the sender's MAC and IP address and the target IP address for which the MAC address is being sought.
  4. ARP Reply:
    • The device with the matching IP address (the target) receives the ARP request and responds with an ARP reply.
    • The ARP reply includes the target's MAC address, and it is unicast back to the original requester.
  5. ARP Caching:
    • To avoid unnecessary ARP requests, devices maintain an ARP cache (also known as ARP table or ARP cache table).
    • When a device successfully resolves an IP address to a MAC address, it stores this mapping in its ARP cache for future use.
  6. ARP Packet Structure:
    • ARP packets consist of fields such as Hardware Type, Protocol Type, Hardware Address Length, Protocol Address Length, Operation Code, Sender Hardware Address, Sender Protocol Address, Target Hardware Address, and Target Protocol Address.
  7. Gratuitous ARP:
    • Devices may also send gratuitous ARP packets to update or announce their MAC address to the network, even if no one has explicitly requested it.
  8. ARP Spoofing:
    • ARP is vulnerable to attacks like ARP spoofing, where a malicious device sends false ARP replies to redirect traffic intended for one device to another. Security measures such as ARP cache validation and dynamic ARP inspection can help mitigate such threats.