web site licensing

hi i need to create simple license for my web site
if user 'a' logged from 'Machine A'
another user can not login with user name 'a' 
if user 'a' logged out from 'Machine A' then user 'a' can login from another machine

if user have 5 lincense they can login 5 entry only 

how to achive this in asp.net web site