ARP (Address Resolution Protocol)

Map IP to MAC address

Translate IP Address to devices physical address (MAC).

Can also be used to get MAC address over LAN (Local Area Network)

Find out what Layer 2 mac address corresponding to layer 3 ip address

By default, when sending the ARP request, the destination MAC address would be 0000.0000.0000 or 00:00:00:00:00:00

ARP Request is a Broadcast whereas ARP Reply is an Unicast

Packet header

1. Hardware Type: The hardware type code, either Ethernet, Serial Link, …
2. Protocol Type: The protocol being used in ARP message. IPv4 is 0x0800
3. Hardware Address Length: MAC address should be 6. Specifies the length in octets
4. Protocol Address Length: Specifies the length in octets of the network-level address. IPv4 should be 4
5. Operation: specifies if it's an ARP Request or ARP reply…

Cisco router debug arp

You can debug arp on cisco router by

debug arp