ARP (Address Resolution Protocol): Understanding IP to MAC Address Resolution

Explore the Address Resolution Protocol (ARP) and its role in translating IP addresses to MAC addresses for local network communication. This guide explains ARP's operation, the ARP cache, ARP requests and replies, and its importance in bridging logical and physical network addressing.

Address Resolution Protocol (ARP) and Its Variants

Introduction to ARP

The Address Resolution Protocol (ARP) is a crucial component of computer networks. It bridges the gap between logical (IP) and physical (MAC) addresses, enabling devices to communicate effectively on a local area network (LAN).

ARP Basics

ARP operates at Layers 2 (Data Link) and 3 (Network) of the OSI model. It translates IP addresses (used for internet routing) into MAC addresses (unique hardware addresses used for communication on a LAN).

The ARP Table and Cache

Each device maintains an ARP table (or cache) that stores mappings between IP and MAC addresses. This cache speeds up communication because subsequent requests for the same IP address don't require a network-wide broadcast.

ARP Request and Reply

When a device needs to communicate with another device but only knows its IP address, it broadcasts an ARP request. The device with the matching IP address responds with an ARP reply, providing its MAC address.

MAC Addresses and Network Interface Cards (NICs)

A MAC address is a unique hardware identifier for a network interface card (NIC). The NIC is responsible for physical network connectivity and uses MAC addresses to send and receive data frames. ARP uses MAC addresses to send the ARP reply.

ARP and IP Addressing/Subnet Boundaries

ARP works within a single subnet. Communication between different subnets requires routers, which handle inter-subnet routing.

Troubleshooting and Monitoring with ARP

Network administrators use ARP tables and caches to troubleshoot network connectivity issues.

ARP Spoofing: A Security Threat

ARP spoofing is a security vulnerability where an attacker sends false ARP messages to redirect network traffic. This can be used for man-in-the-middle attacks or denial-of-service attacks.

ARP Security Measures

Protecting against ARP spoofing includes techniques like static ARP entries (manually configuring IP-to-MAC mappings) and monitoring for ARP cache poisoning.

Types of ARP

Besides standard ARP, several variants exist:

  • Proxy ARP: A router responds to ARP requests for devices on other subnets. This allows communication across subnets without explicit routing configuration.
  • Gratuitous ARP: A device announces its IP-MAC address mapping to update the network's ARP cache (useful when adding a new device or changing an IP address).
  • Reverse ARP (RARP): Used by diskless workstations to obtain their IP addresses from their MAC addresses.
  • Inverse ARP (InARP): Used in Frame Relay networks to map IP addresses to data link layer addresses (DLCIs).

Uses of ARP

  • Efficiently resolves IP addresses to MAC addresses.
  • Automatic and dynamic operation.
  • Widely supported.
  • Scales well to larger networks.
  • Supports dynamic IP address allocation (DHCP).

Drawbacks of ARP

  • Vulnerable to spoofing and cache poisoning.
  • Limited to single subnets.
  • A corrupted ARP cache can disrupt network communication.
  • Lacks built-in authentication.
  • ARP broadcasts can cause network congestion.
  • Less straightforward integration with IPv6 (Neighbor Discovery Protocol is used instead).