Till now we have discussed the two main categories of routing protocols for MANETs, namely, proactive and reactive protocols. However, there are other protocols that cannot be classified as purely reactive or purely proactive. We discuss two such important classes of protocols in this section, namely, the zone routing protocol (ZRP) and link reversal routing protocols.
The zone routing protocol  takes advantage of both proactive and reactive routing strategies in a MANET. Recall that the main drawback of proactive protocols is their large overhead due to proactive exchange of routing tables or link state information and that of reactive protocols is their high latency. ZRP tries to overcome both of these drawbacks while preserving the advantages of both proactive and reactive protocols.
Each node N executing the ZRP in a MANET maintains its routing zone. A routing zone is a k-hop neighborhood of N, where k is usually a small number from 2 to 4. k is also called the radius of a routing zone and is same for all nodes in the MANET. Each node routes proactively within its routing zone, that is, it tries to maintain a complete routing table for all the nodes in its routing zone, like the DSDV protocol. The routing outside this routing zone is done reactively, like the DSR or AODV protocol.
Suppose node S wants to start a data communication with node D. If D is within the routing zone of S, then there is no need for route ...