Address Resolution Protocol (ARP)

https://www.youtube.com/watch?v=NpiORFxyM4c

• ARP is a protocol for mapping an IP address to a physical MAC address on a local area network.
• Program used by one device to find another device's MAC address based on that device's IP
• Device's maintain ARP Cache tables mapping IP to MAC
  • arp -a lists table

How does ARP work?

• Client (192.168.1.10) wants to communicate with Server (192.168.1.50)
• Client knows Server has IP of 192.168.1.50 and is local.
  1. Client broadcasts packet to all IPs:
     • "Are you 192.168.1.50? Please send MAC"
       • IP Source: 192.168.1.10
       • MAC Source: oe:cd:ef:12:34:56
       • IP Dest: 192.168.1.50
       • MAC Dest: ff:ff:ff:ff:ff:ff (broadcast)
     • For each device that 'hears' broadcast: if not 192.168.1.50 silently ignore packet
  2. Server 'hears' packet - sends Unicast (1-to-1 communication) to Client
     • IP Source: 192.168.1.50
     • MAC Source: fa:ed:db:91:11:19
     • IP Dest: 192.168.1.10
     • Mac Dest: oe:cd:ef:12:34:56 (unicast)
  3. Client confirms by send Server a request
     • IP Source: 192.168.1.10
     • MAC Source: oe:cd:ef:12:34:56
     • IP Dest: 192.168.1.50
     • MAC Source: fa:ed:db:91:11:19 (unicast)
     • Client updates ARP Cache table for future reference

ARP Summary

1. Layer 2 protocol (aruably as Layer 2.5 as it exists between layers)
   • Uses Layer 3 IP address to find Layer 2 MAC address
2. Operates on LAN (same broadcast domain)
   • Relies on broadcasting
3. Uses/Updates ARP Table