Monday, September 24, 2012

ARP Cache/Table

The Address Resolution Protocol or ARP is very important in data network communication. It is used to translate Layer 3 address (IP) to Layer 2 address (MAC). To do so, host or switch/router will create one ARP table which contains the mapping between IP address and MAC address.

Like MAC table, ARP table could be statically configured and dynamically learned. ARP protocol is used for learning. Basically, when the host wants to send IP packets to 192.168.100;  It will first query whether corresponding entry in ARP table. If found,  the corresponding MAC address will be the L2 address used. Or else,  the host will broadcast one ARP request saying: whose IP address is, and what's your MAC address? Every host on the same LAN will compare its IP address with the request; If they matches, it will respond with its own MAC address. Then the host will update its local ARP table for future usage.
To prevent ARP table becoming too big, ARP table entry will age out. The aging time could be configured in
modern operating system or switch/routers.

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