how to use the same var on each page with different value that the master page knows

i have a menu and i need to know which page i am on to mark the relavent menu button as "on".
i am using a master page (thats where the menu is) and i was thinking if on each page i have a var pageNum and then on the master page i could use that var to know who i am.
how do i do it? how do i set the var in each page and also have the master page read it and use it on page load
please i need the code


thank you
natalie



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