hi. i am using ubuntu 12.04 and trying to install NS2.35
for that matter i downloaded ns-allinone-2.35, unzipped it, started installation, the installation went well until it gave an error message and stopped. the error message is as follows:

: fatal error: X11/Xlib.h: No such file or directory
compilation terminated.
make: *** [tk3d.o] Error 1
tk8.5.10 make failed! Exiting ...

I'm pretty sure that I do have Xlib.h at the right place. i also tried to do this 'sudo apt-get install libx11-dev' but it returned me the following:

"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


in desperate search to an immediate solution.