Installing NS2:

In ubuntu, there are two ways of installing NS2.

First to to download ns-allinone2.35 or the latest version at http://sourceforge.net. Direct link to
NS-2.35 is http://sourceforge.net/projects/nsnam/.

Extract the downloaded file into a folder and go to .../ns-allinone-2.35/ns-2.35/allinone folder, in which "install" file will be there. Double click it and run in terminal.
Terminal is opened in ubuntu by cntr+alt+t
Second way is to open terminal and type:

sudo apt-get install ns2
sudo apt-get install nam
sudo apt-get install xgraph

After this type ns in your Terminal, if you don't end up with a error command then you are ready to have fun with NS2.

The installation is enough for basic simulation, but for advanced simulation, we need to install C++, using sudo apt-get install g++ command in terminal.

In windows, NS2 is used using Cygwin.

To install, download ns-allinone-2.35 or latest version as discussed and go to  ../ns-allinone-2.35/ns-2.35/allinone where INSTALL.WIN32 is present. Install it in cmd window.

Network Simulator:(NS-2 and NS-3)

Simulating live networks is never been this easy. Now we can know peek inside the network to know about it just with the help of your system. Network simulator was developed with same idea I guess.

First two of its versions, i.e.. NS and NS2 are bit successful in terms of simulating a network, but are failure when it comes to speed and complexity. Their complexity is mainly because of their double layer architecture using Tcl and C++ as outer and inner layers respectively. These two are linked with Tclcl class. Too much Technical I guess, :P. To know more about them, I recommend you to go through http://www.isi.edu/nsnam/ns/ and http://www.nsnam.org. It has tutorials and everything you want to know about Network simulators. Some tutorials may be confusing or not elaborate enough, but you will get a fair outer idea about network simulators. Whatever the reason may be NS and NS2 are slow. NS3 was able to overcome this. When we look at their names, we may think they are just developments from one to another, but Surprisingly, NS3 is not even similar to previous versions. In NS3 coding is solely done by C++, optionally by Python. If you are new to this, I prefer you to start with NS3. Everything we talked about is open source. So your contribution may matter.

Links you may be interested in

Installing NS2

Installing NS3