assign valid ip

hi

any body knows how can i assign valid ip to the vpn server? i mean when anyone try to connect to vpn server use that ip

i have tmg 2010 and i config VPN Clients Tasks & General VPN Configurationjust like this

http://www.isaserver.org/articles-tutorials/configuration-general/Checking-Out-TMG-2010-Virtual-Private-Network-Server-Part1.html