show navigation on certain pages only?

Hi Everyone,

Im currently building my theme and one of my pages is portfolio, My current navigation bar shows Home, About, Portfolio and Contact; However what I would like to implement is the additional portfolio pages I plan on creating, As I create these pages they are automatically added to my navigation, Is there a way to stop them appearing across the site and only appear once on the navigation page? Or is there a plugin I can use to recreate this effect? Thanks!