Imagine the following scenario: there is a big building, with hundreds of apartments, and every apartment uses the same ISP. Each apartment has their own router.
If a website logs every IP address, is it likely that two different apartments accessing the website will result in the same IP being logged?
In other words, assuming nobody changes their IP manually with some trick, TOR, etc., how safe it is to assume that two connections with the same public IP to my website come from the same apartment?
42 Answers
If they're behind Carrier-grade NAT they will all have the same IP address. Otherwise it's the same chance as any two random customers of that ISP.
Basically, you cannot use IP address as a reliable identification method.
2If two connections come from a single public IP, all you know without further information is that they probably share a router.
Any of the following is possible; the two connections can:
come from the same computer,
come from different computers in the same house,
come from different apartments in a building that shares a router,
come from a wide number of possible people behind a CGNAT setup,
come from the same or different computers that are sharing the same public Wifi access point,
come from a hacked or purposefully-configured system that is proxying for other computers anywhere else in the Internet.
IP is a dumb protocol by design so it can operate as fast as possible, capacity added as flexibly as possible, and be bypassed/shunted elsewhere as fast as possible in the event of problems.
IP addresses are not meant to identify people, but interfaces.
You need additional mechanisms to authenticate and secure IP based transport, and if an IP network is accessible to the public, prepare for abusive situations you can't place blame at specific individuals immediately--like DDoS, etc.
