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The hosts are assigned IP address by the router using dhcp. So, the router has a list (mac addresses) of all the hosts that are directly connected to it as well their ip addresses. Now if a packet comes from outside, the router must be able to find the destination mac address from its logs. Then what is the need to use ARP in such a case (hosts connected to router)?

The only time I see an use of ARP is when router has to find new routers. Using ARP allows it to find new routers as well as to force a more rigid use of longest matching address method if needed. However what is the use of ARP during "router to host" packet transfer?

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  • $\begingroup$ there are some sophisticated algorithms running in routers to handle "adjacent router" tables, but this is maybe more of a networking question, see eg understanding arp routers / Network Engineering $\endgroup$
    – vzn
    Jan 13, 2017 at 16:19

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The fact that the router has the list does not help when computer A wants to talk to computer B, because neither of the two has the list.

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