when u talk on phone, do u also speak ur phone number? no right?
but still receiver can see ur number (CLIP) because exchange knows which wire the voice packets are coming from.
And then exchange attaches CLIP info to a voice call when it forwards your call to another exchange.Otherwise, next exchange will be clueless abt caller.
Extending this to Port Binding:
PPP packets land at DSLAM port, which is attached to a phone line.
If DSLAM does not attach port information here, PPPOE Access Concentrator [Auth Server] will be clueless about the DSLAM port from which it received the request.
The way it is done is, iSMART will DELETE username info and replace it with actual telephone number [It will be Home-Port for this line].
If your telephone number is 234544, PPPOE server will see 234544 as username even if you enter something else.
That ways you will not be able to connect to someone else's account.
Three things:
1. It is not a fool-proof thing since iSMART can be left off on certain ports by manual error. These lines can login as anyone else if they know the password.
2. iSMART is NOT the only option.
3. Even if port-binding is off, phone number can be traced by looking at ATM frames [some layers below TCP/IP].