How do ISPs know what IP you have?

[zENGER]

My brother is an admin at an ISP. His boss is the anti-gov't type and has the logs purged as soon as they're created to remove the possibility to track you. I would assume some other ISPs might be the same.

Don't expect because you're getting ARP packets that the network has forgotten about you. Its just checking to make sure you're still there and still the same person/machine.

If you spoofed the MAC of another customer and that customer came on, and the ISP is not using a layer three switch, you'll have some really weird results.

If you try to bypass the DHCP servers you could get an IP collision when the DHCP server decides to hand out the address that you have. You can't just make up an address that the server would not hand out, because most likely the only routable IPs on the network are going to be in the DHCP pool.

You also have to worry about authentication. ISPs don't just allow anyone to connect to their network. You either have to have a login/password which could be used to track which connection was yours. If you have a broadband connection that doesn't require a login, they can still track you. I'm not positive how this works, but I do know that if you are having a problem and call tech support for the company they can figure out which device is yours and test it. This might have to do with the MAC of the cable/dsl modem which is most likely hard to change, or they might have a way of telling which pipe runs to your house. This part I'm not sure exactly how it works, but I know they can tell which is which.

[cacosapo]

Originally posted here by zENGER
His boss is the anti-gov't type and has the logs purged as soon as they're created to remove the possibility to track you. I would assume some other ISPs might be the same.

I have no idea how it works on USA, but here if an ISP doesnt keep log files to show to the COPS..... police can charge ISP owner/admins with the same crime as the "user" did.