In a computer network, the Next Hop Resolution Protocol (NHRP) is a protocol or method that can be used so that a computer sending data to another computer can learn the most direct route (the fewest number of hops) to the receiving computer. If the receiving computer is in the same subnetwork, the use of NHRP will tell the sending computer that the receiving computer is local and it can send subsequent data packets directly to the receiving computer using its subnetwork address rather than its global network address. If the receiving computer is not in the same subnetwork, the use of NHRP will tell the sending computer the computer in the subnetwork whose router provides the most direct path to the receiving computer and the sender can now forward subsequent data packets to that router.
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NHRP is basically a query-and-reply protocol and all parties through which reply information passes build a "network knowledge table" that can be used for all subsequent traffic. Using this knowledge, a computer can send subsequent packets directly to the destination computer (or to an egress router) using an acquired "machine" address rather than a network address. (This kind of operation at the Media Access Control layer of the Data-Link layer communication layer level - rather than at the Network layer - is sometimes referred to as "operating at close to wire speed" because fewer program instructions are required and these can sometimes be located in a switching device.) NHRP thus provides two benefits: it reduces the number of hops a packet may have to take within a subnetwork and it allows the packets to be forwarded using the faster "machine" address.
NHRP was developed by the Internetworking Over NBMA working group of the Internet Engineering Task Force (IETF) . It is a main component of Multiprotocol over ATM (MPOA) and is described in the IETF's RFC (Request for Comments) 2332, "NBMA Next Hop Resolution Protocol."