Overview of ARP

Definition

Address Resolution Protocol (ARP) maps IP addresses to MAC addresses. ARP entries are classified as static and dynamic ARP entries. In addition, ARP provides extension application functions, such as proxy ARP and gratuitous ARP.

Objective

Each host or device in a local area network (LAN) has a 32-bit IP address for communicating with other hosts. IP addresses are independent of hardware addresses. On an Ethernet, a host or a router transmits Ethernet frames based on 48-bit MAC addresses. A MAC address is also called a physical or hardware address. It is allocated to an Ethernet interface when a device is produced. In actual networking, MAC and IP addresses must be mapped using an address resolution mechanism.

ARP supports the following functions:

• Dynamic ARP

  ARP dynamically resolves an IP address into an Ethernet MAC address based on ARP packets. No network administrator interference is required.

• Static ARP

  Static ARP establishes a fixed mapping between the IP and MAC addresses, which cannot be dynamically adjusted on a host or router. Network administrator interference is required.

• Proxy ARP

  Also called routed proxy ARP. If a host is not configured with a default gateway address, the host can send an ARP Request packet to request the destination host MAC address. After the device enabled with proxy ARP receives the packet, it sends an ARP Reply packet containing its own MAC address so that internal hosts on different physical networks but on the same network segment can communicate.

• Gratuitous ARP

  Gratuitous ARP checks existing IP addresses and declares new MAC addresses.

• Authorized ARP

  Authorized ARP, valid on only devices enabled with the DHCP server function, applies when the DHCP server and DHCP client reside on the same network segment to prevent attackers from forging the IP addresses or MAC addresses of legitimate DHCP clients to launch attacks.