What is arp protocol?

ARP is the protocol which is used to resolve address mapping between L3 address and layer2 addresses in LAN (For Communication on LAN Networks, we require Hardware / MAC Address). For Instance, PC1 on LAN Requires to Communicate with PC2 on the same LAN Network. It Does know it has to communicate with1.1.1.1 IP address. But which PC actually has1.1.1.1 on the LAN is unknown to PC1. So It would say I need to communicate with1.1.1.1 who is that one (Asking for MAC Address) the PC2 will reply back I am1.1.1.1 and my MAC Address is0100.0100.0100. I hope this helps.

simply arp (address resolution protocol) is a protocol used to resolve ip address to get MAC address for the host 

another protocol is rarp  is a reverse of arp it use MAC address to get the IP

ARP ( @ resolution prptocol ) is a protocol used to translate IP @ into MAC  @

ARP is also know as Address Resolution Protocol

Definition: ARP converts an Internet Protocol (IP) address to its corresponding physical network address. ARP is a low-level network protocol, operating at Layer2 of the OSI model.

ARP usually is implemented in the device drivers of network operating systems. 

ARP works on Ethernet networks as follows. Ethernet network adapters are produced with a physical address embedded in the hardware called the Media Access Control (MAC) address. Manufacturers take care to ensure these6-byte (48-bit) addresses are unique, and Ethernet relies on these unique identifiers for message delivery. When any device wishes to send data to another target device over Ethernet, it must first determine the MAC address of that target given its IP address These IP-to-MAC address mappings are derived from an ARP cache maintained on each device. If the given IP address does not appear in a device's cache, that device cannot direct messages to that target until it obtains a new mapping. To do this, the initiating device first sends an ARP request broadcast message on the local subnet. The host with the given IP address sends an ARP reply in response to the broadcat, allowing the initiating device to update its cache and proceed to deliver messages directly to The target.

The following is a sample of output from the show ip arp command:

Router#show ip arp

Protocol AddressAge(min) Hardware Addr Type InterfaceInternet171.69.233.2290000.0c59.f892 ARPA Ethernet0/0Internet171.69.233.2180000.0c07.ac00 ARPA Ethernet0/0Internet171.69.233.19-0000.0c63.1300 ARPA Ethernet0/0Internet171.69.233.3090000.0c36.6965 ARPA Ethernet0/0Internet172.19.168.11-0000.0c63.1300 ARPA Ethernet0/0Internet172.19.168..0c36.6965 ARPA Ethernet0/0

to know the complete detail.

Syntax: show ip arp detail.

Definition: ARP converts an IP to its corresponding physical network address. ARP is a low-level network protocol, operating at Layer2 of the OSI Model.

Address Resolution Protocol (ARP) is used to map ip addresses to MAC addresses. The opposite of it is done by Reverse Address Resolution Protocol (RARP).

What is ARP: Is used to translate an IP address into MAC address

When it is Used: In  Layer2 communications between networked systems, IP address is not used. So, within a LAN segment computers identify and communicate with each other using the MAC Address

Types of ARP messages:

Unicast: If MAC address is present in ARP Cache

Broadcast: If MAC address is not present in its ARP cache table for corresponding IP address

Method of ARP working:

When a computer  A wants to send a Packet to computer B, First computer A  uses a cached ARP table to look up for Computers B IP address for any existing records of  MAC address. If the Mac address is found, it sends the IP packet on the link layer to Computer’s B Mac-address. If the cache did not produce a result for Computer’s B IP address , Computer A has to send a broadcast ARP message (destination FF:FF:FF:FF:FF:FF MAC address which is accepted by all computers) requesting an answer for Computers B IP address. Computer B responds with its MAC address (and its IP). The response information is cached in Computer’s A ARP table and the message can now be sent.