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What is ARP?

Address Resolution Protocol. A protocol that finds the MAC address of a device on the same network from its IP address. The request is broadcast, and the matching device replies.

Japanese name: ARP

ARP appears in 2 practice questions across 1 topic, including Computer Networking Basics.

Field: Computer Networking Basics

Try it: questions about ARP

Which correctly describes the role of ARP?

  1. AIt finds the MAC address of a device on the same network from its IP address
  2. BIt finds a server's IP address from its domain name
  3. CIt automatically assigns IP addresses to devices that join the network
Show answer and explanation

Answer: A. It finds the MAC address of a device on the same network from its IP address

In ARP (Address Resolution Protocol), the host that wants to send broadcasts a request saying "whoever has this IP address, tell me your MAC address," and only the matching device replies with its MAC address by unicast. The resulting mapping is kept in the ARP cache for a while and reused. Because Ethernet delivers frames by MAC address, this mapping is needed before an IP packet can be sent. Finding an IP address from a domain name is the job of DNS, and assigning IP addresses automatically is the job of DHCP. IPv6 uses ICMPv6 Neighbor Discovery (NDP) instead of ARP.

A host (192.168.1.10/24, default gateway 192.168.1.1) sends a packet to server 203.0.113.80 on a different network. With an empty ARP cache, whose MAC address does the host look up with ARP?

  1. AThe default gateway, 192.168.1.1
  2. BThe server, 203.0.113.80
  3. CEvery router on the path to the server
Show answer and explanation

Answer: A. The default gateway, 192.168.1.1

The host first uses its own IP address and subnet mask to decide whether the destination is on the same network. 203.0.113.80 is outside 192.168.1.0/24, so the packet is handed to the default gateway (a router). The destination in the IP header stays the server's 203.0.113.80; only the frame's destination MAC address is set to the gateway's. So ARP looks up the MAC address of 192.168.1.1. ARP broadcasts do not cross routers, so the host cannot look up, and does not need, the MAC address of a server on another network. Each router along the path looks up the MAC address of the next device whenever it forwards.

ARP in the explanations

Questions that mention ARP (2)

  1. Which correctly describes the role of ARP?
  2. A host (192.168.1.10/24, default gateway 192.168.1.1) sends a packet to server 203.0.113.80 on a different network. With an empty ARP cache, whose MAC address does the host look up with ARP?

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Related terms

MAC address DNS (Domain Name System) DHCP Default gateway Subnet mask CIDR CSMA/CA NAT (Network Address Translation) OSI reference model Port number

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