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ARP

ARP (Address Resolution Protocol) is a fundamental protocol used in the Internet Protocol (IP) suite for finding the media access control (MAC) address corresponding to a given Internet Protocol (IP) address. Its primary purposes include:

  1. Resolving IP Addresses to MAC Addresses: ARP translates 32-bit IP addresses to 48-bit MAC addresses. This translation is necessary because while IP addresses are used for routing packets over an IP network, the actual data link layer (like Ethernet) relies on MAC addresses for delivering packets on the same network.

  2. Facilitating Communication within a Network: For a device to send data to another device on the same local network, it needs to know the recipient's MAC address. ARP enables this by mapping the network layer addresses (IP addresses) to data link layer addresses (MAC addresses).

How ARP Works​

  1. ARP Request: When a device (say, Device A) wants to communicate with another device (Device B) on the same local network, it first checks its ARP cache to see if it already has Device B's MAC address. If not, it broadcasts an ARP request packet to all devices on the local network. This packet contains Device B's IP address and asks which MAC address it belongs to.

  2. ARP Reply: Device B, upon receiving this ARP request and recognizing its IP address in the message, sends an ARP reply back to Device A. This reply contains Device B's MAC address.

  3. Updating ARP Cache: Device A receives the ARP reply, updates its ARP cache with the new IP-to-MAC address mapping (to avoid future ARP requests for the same address), and can now send data to Device B using the MAC address.

ARP Cache​

  • ARP Table/Cache: Devices maintain a table of IP-to-MAC address mappings known as the ARP cache. These entries are stored for a certain period to minimize the number of ARP requests.

ARP in Different Network Types​

  • In Ethernet networks, ARP is used for mapping IP addresses to Ethernet MAC addresses.
  • In Wi-Fi networks, ARP operates similarly, mapping IP addresses to Wi-Fi MAC addresses.

Security Considerations​

  • ARP Spoofing/Poisoning: ARP does not have a built-in mechanism to verify the identity of the sender or receiver. This vulnerability can be exploited in ARP spoofing attacks, where a malicious actor sends fake ARP messages onto the network, linking their MAC address with the IP address of another device (usually a gateway).

Conclusion​

ARP plays a crucial role in the functionality of IPv4 networks, bridging the gap between the logical addressing used by IP and the physical addressing used by the data link layer. While essential for network communication, its lack of authentication measures requires additional security considerations to prevent ARP spoofing attacks. In IPv6, ARP's functionality is replaced by the Neighbor Discovery Protocol (NDP), which includes security features not present in ARP.