Address Resolution Protocol

Overview

Address resolution protocol (ARP) is a protocol for address resolution. Though it general enough for use with a wide variety of address types, it it almost always used to map between 32-bit IPv4 addresses and MAC addresses.

Frame Format

The following is a diagram of the ARP frame format.

• Destination (DST). The destination MAC address.
  • The special MAC address ff:ff:ff:ff:ff:ff is used in ARP requests to indicate the broadcast address.
• Source (SRC). The source MAC address.
• Length or Type. Set to 0x0806 for both ARP requests and replies.
• Hardware Type (Hard Type). Specifies the type of hardware address. Usually 1 for Ethernet.
• Protocol Type (Prot Type). Specifies the type of protocol. Shares a numbering space with those used in Ethernet's length/type field.

Protocol Operation

ARP works by sending an Ethernet frame called an ARP request to every host on a shared link-layer segment. This is called a link-layer broadcast. The ARP request contains the IPv4 address of the destination host. If a station using the IPv4 address receives the request, it alone sends an ARP reply containing the IPv4 address and the corresponding MAC address.