about portal security

[info () biledge com]

hi,

i need to secure a web portal with 7,000 members. certain pages will be extranet 
and i am not sure if i will need to secure them separately. 
if i have security for the portal, does that mean i have security for the extranet 
part of the portal too ?
thank you for the helps, thank you for no helps too :)
regards,
bilur