ARP resolves MAC address from IP address

A PC in an Ethernet network can communicate with another PC, if only it knows the Ethernet address (MAC address) of that PC. Address Resolution Protocol (ARP) is a network protocol uses to translate a PC's IP address to physical (hardware) address. From OSI model point of view, ARP belongs to the OSI data link layer (Layer 2).

When a PC to resolve a given IP address to Ethernet address, an ARP request packet is broadcasted to the whole network. This ARP request packet contains the source MAC address, the source IP address and the destination IP address. Every PC in the local network receives this packet - whether it is connected to a hub or switch as this is a broadcast packet. This ARP request is then processed by all the PCs in the network and the PC with the specified destination IP address will send an ARP reply packet to the originating host with its MAC and IP address.

ARP maintains the mapping between IP address and MAC address in a table in memory called ARP cache. To display the current ARP entries, execute arp command with "-a" option.

C:\arp -a

Interface: 202.156.207.116 --- 0x2
Internet Address Physical Address Type
202.156.200.1 00-30-b8-c7-d9-90 dynamic

The entries in the ARP cache are dynamically added and removed.
Use -d option to arp command to clear the ARP table.

C:\arp -a *

To learn more about ARP such as packet format, refer to RFC 826.