To install The Network Simulator - ns-2 we need to bit of circus.

Install dependencies :
$ sudo apt-get install libpcap-dev gcc-4.3 g++-4.3 tcl  tk  tk8.4-dev tcl8.4-dev xorg-dev sgb

By default gcc version is 4.4, but has ns2 compiles well only will 4.3 we need to downgrade gcc !

Backup the current links and update symbolic links :

$ sudo mv gcc gcc.old
$ sudo mv i486-linux-gnu-gcc i486-linux-gnu-gcc.old 
$ sudo ln -s gcc-4.3 gcc 
$ sudo ln -s gcc-4.3 i486-linux-gnu-gcc 

Download and Install ns2:

$ wget http://sourceforge.net/projects/nsnam/files/allinone/ns-allinone-2.33.tar.gz/download
$ sudo tar zxvf ns-allinone-2.33.tar.gz
$ sudo cd ns-allinone-2.33 
$ sudo ./install 

P.S : In case of nay issues with gcc or g++ restore the backup