Zone Routing Protocol (ZRP) was first introduced by Haas and Pearlman [25]. It is a hybrid protocol. To perform operations it divides the total network area into different zones. Zone size or radius does not depend on the distance; it depends on the number of hops. It is applicable in a wide variety of mobile ad-hoc networks with diverse mobility across a large span. It uses separate strategy to find out new routes for nodes which are lying within or outside the zone. There are four elements available in ZRP: MAC level function, IARP, IERP and BRP. IARP, proactive protocol is used to discover route within zone and in this case, links are considered as unidirectional. But in order to communicate with the nodes which locate in different zones, nodes use IERP, on-demand routing protocol. ZRP also follows different strategies, such as routing zone topology and proactive maintenance, for improving the efficiency and quality to discover a globally reactive route using query/reply mechanism [13].
The ZRP has versatile properties and applications. Zone radius is an important parameter of ZRP. A large routing zone is more suitable for slowly moving nodes and high demand of route scenarios. In fixed topology, network zone would be infinitely large. In fixed internet, pure proactive routing protocols are best suited. Smaller routing zone is suitable for minimum nodes and where demand of route is low. ZRP works as a normal flooding protocol when zone size is one. In order to identify the direct neighbor nodes, it uses MAC protocol. And to identify the other nodes within the zone it uses NDP (Neighbor Discovery protocol) [14].