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title="Permalink to What is ARP : A quick overview">What is ARP : A quick overview</a></h2>
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<p>LAN switches and network interface card fitted on each station understand MAC address. On the other hand, an application running on a station knows the IP address of the party it wishes to communicate with. If two parties, A and B, on the same LAN wish to communicate, they should know each other's MAC address also, not just their IP address.</p>
<p>The Address Resolution Protocol(ARP) is used to resolvean IP address to a MAX address. If A wishes to initiate communication with B, it should know B's address. If it does not, A broadcasts a special ARP query reques frame wherein it poses the following query </p>
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<p><em>What is B's MAC address ?</em></p>
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<p>The query is recieved by all nodes on the LAN. If B is live on the LAN, it sends a unicast response to A as follows:</p>
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<p><em>B's MAC address is 0A:1B:2C:3D:4E:5F:99</em></p>
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<p>The mapping between B's IP address and its MAC address is then cached at A for future use in its local ARP cache. All entries have an expiry time. So, LANs typically see a fair amoutn of ARP traffic since all the stations need to refresh their expired cache entries.</p>
<p>There are a number of note worthy features of the ARP protocol. Two of the most relevant (from a security perspective) are as follows.
- The ARP request frame also contains the MAC address and the IP address of the sender A. Being a broadcast packet, all stations on the LAN recieve it. On receipt of the query frame, they all update their ARP caches with the MAC address of A.
- The ARP protocol permits any station to send or recieve and cache unsolicited ARP replies.</p>
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<span class="post_date">Wed 18 October 2017</span>
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