NS2 is the most popular open source Network Simulator. NS2 is an object-oriented, discrete event driven network simulator that mainly finds its application in the networking related academic researches to simulate wired and wireless networks. It uses C++ and OTcl (a Tcl script language with Object-oriented extensions) as its programming language. Tcl is a command line tool C++ script executes any of the networking protocols whereas OTcl script enables the users to manage the simulation scenario and schedule the events. OTcl supports certain features that C++ lacks. Therefore, NS2 couples these two scripts to improve its efficiency.
Network Simulator Version 2, also known as NS-2. NS-2 is an event driven packet level network simulator developed as part of the VINT project (Virtual Internet Testbed). Numerous state of the art simulation tools are available for network research. Among the available simulators, NS-2 is the widely used and highly recognized open source network simulator. The simulation behavior of NS-2 is highly trusted within the research community. It is a dependable and realistic discrete event based simulator tool and proves to be more practicable. It is primarily designed based on OSI model to support wired networks. CMU wireless extension of NS2 extensively supports wireless networks.