ARP (Address Resolution Protocol): Mapping IP Addresses to MAC Addresses

Understand how the Address Resolution Protocol (ARP) works to map IP addresses to MAC addresses, enabling communication between devices on a local area network (LAN). This guide explains the process, its role in the OSI model, and its importance for local network communication.

Address Resolution Protocol (ARP): Mapping IP Addresses to MAC Addresses

Introduction to ARP

The Address Resolution Protocol (ARP) is a crucial networking protocol that helps devices on a Local Area Network (LAN) find each other. Specifically, it translates an IP address (a logical address used for internet routing) into a MAC address (a physical address unique to each network interface card). This is necessary because, while IP addresses are used for routing data across larger networks, communication at the physical level (within a LAN) requires MAC addresses.

How ARP Works

ARP's main function is to convert a 32-bit IP address (IPv4) into a 48-bit MAC address. This process occurs between Layers 2 (Data Link Layer) and 3 (Network Layer) of the OSI model. ARP is used only when both the source and destination devices are on the same network.

ARP Process: An Example

Let's say Device A wants to communicate with Device B. Device A knows Device B's IP address but needs its MAC address:

  1. Check ARP Cache: Device A first checks its own ARP cache (a table storing known IP-to-MAC address mappings) to see if it already knows Device B's MAC address. If found, it uses this information.
  2. ARP Broadcast: If Device B's MAC address isn't in the cache, Device A broadcasts an ARP request to the network, asking "Who has this IP address?"
  3. ARP Reply: Device B (which has the matching IP address) responds with a unicast message (directed only at Device A), providing its MAC address.
  4. Cache Update: Device A adds the new IP-to-MAC mapping to its ARP cache.

Types of ARP Mapping

  • Static Mapping: The IP-to-MAC address mappings are manually configured in the ARP table. This is useful for static IP addresses.
  • Dynamic Mapping: The MAC address is determined dynamically by sending an ARP broadcast request. These entries are temporary and are periodically removed from the ARP cache.

(Note that the Reverse Address Resolution Protocol (RARP) uses dynamic mapping to obtain an IP address from a MAC address.)

Conclusion

ARP is a fundamental protocol for local network communication. By dynamically resolving IP addresses to MAC addresses, it ensures that data reaches the correct devices, simplifying the process of network communication at the data link layer.