If the NS is to be the AP what is the task of your PC?
If your PC running ROS is attached to an ISP by a modem or router, you probably need a second Ethernet port in your PC to connect it to the NS.
If you just want to grand internet connectivity to the NS-AP than set the ROS-PC up as bridge so NS as dhcp-client or PPoE client will get the authentication and IP address etc. from ISP.
But there are many other ways to do it. So you really need to give us more info on what your really want because basically you can setup the NS2 to connect directly to adsl-modem or router for instance.
For questions about how to setup the NS2 go to the ubnt forum.
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Rudy R. Puister
WISP operator based on MT routerboard & ROS.