remote access on different networks

[Gerline Fleury]

   Hi  I  am  trying  to  access  both my computer at work, and a friends
   computer using VNC.  When I brought my laptop to my friends house, and
   connected  to  her internet system I was able to use VNC to access her
   computer.   But once I left and used my home internet, I was no longer
   able to access her system.

   I  am  confused.   I  was  under  the impression that with VNC you can
   access  another computer from anywhere as long as the computer you are
   trying to access has given you permission.

   What can I do to ensure that  will be able to access another computer,
   even  if  I  am  not  on  the  same  internet  connection as the other
   computer?