The MANET stands for mobile ad hoc network; in practice, the term generally applies to ad hoc wireless networks of sufficient complexity that some internal routing mechanism is needed to enable full connectivity.
The term mesh network is also used for MANETs.
MANET nodes communicate by radio signals with a finite range, as in the Figure - 1 below.
Each node's radio range is represented by a circle centered about that node. In general, two MANET nodes
may be able to communicate only by relaying packets through intermediate nodes, as is the case for nodes
A and G in the diagram above.
Mobile Ad Hoc networks can use any wireless mechanism
In the field, the radio range of each node may not be very circular, due to signal reflection. An additional complication arises when the nodes (or even just obstructions) are moving in real time (hence the “mobile” of MANET); this means that a working route may stop working a short time later.
For this reason, routing within MANETs is a good deal more complex than routing in an Ethernet. A switched Ethernet, for example, is required to be loop-free, so there is never a choice among multiple alternative routes.
MANETs in general do not support broadcast, unless the forwarding of broadcast messages throughout the MANET is built in to the routing mechanism.
MANETs are of theoretical interest, their practical impact has been modest; they are almost unknown, for example, in corporate environments.
They appear most useful in emergency situations, rural settings, and settings where the conventional infrastructure network has failed or been disabled.
RFC 5820 specifies the needs for OSPF to run on Mobile Ad Hoc Networks. There are many extensions for OSPF such as flooding enhancements, packet structure and so on.
Some MANET use cases are depicted below.
Figure - 2 Mobile Ad Hoc Networks in military
Figure - 3 Mobile Ad Hoc Network in Public Environment