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What is Address Resolution Protocol-ARP

ARP (Address Resolution Protocol) is a Layer2 protocol that is used to translate IP addresses (layer 3 in OSI model ) to MAC addresses (layer 2 in OSI model ), and is used when two hosts on a Local Area Ethernet Network want to communicate with each other. Of course ARP is used in other Layer 2 networks such as ATM for example, but the main usage of ARP is on Ethernet Networks.

The first time Host A wants to contact Host B, Host A sends an ARP Request (using a broadcast packet) on the LAN. The ARP packet basically asks the following “I have the IP address for Host B and I want to know the MAC address of Host B”.

Host B sees this broadcast packet and replies to Host A with an ARP Reply packet containing host B’s MAC address. Host A now saves this MAC address/IP address combination in its ARP cache and begins to communicate with host B. This communication happens with Layer2 MAC Addresses because the two hosts are on the same Layer 2 network.

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To display the contents of a host’s ARP cache (table), use “arp-a” on a Windows platform, “arp” on a UNIX platform, and “show arp” on a Cisco IOS platform.

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Filed Under: CCNA Training Tagged With: address resolution protocol, arp

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About Networks Training

We Provide Technical Tutorials and Configuration Examples about TCP/IP Networks with focus on Cisco Products and Technologies. This blog entails my own thoughts and ideas, which may not represent the thoughts of Cisco Systems Inc. This blog is NOT affiliated or endorsed by Cisco Systems Inc. All product names, logos and artwork are copyrights/trademarks of their respective owners.

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