I am new to ubuntu, I am using ubuntu 13.10, and trying to install NS-2.

During the installation I am facing the following error;

X11/Xlib.h: No such file or directory

Then i tried to install

sudo apt-get install libx11-dev

This time I am facing the following error;

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package libx11-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'libx11-dev' has no installation candidate

I have no idea how to deal it, Please help Thanks in advance