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I assume "shared IP addresses" work by NAT, because obviously two devices with the same IP address need someway to distinguish them.

I've seen some privacy advocates boast they have "shared IP addresses" making it impossible for someone to track their activity.

For example

IPVanish takes an interesting approach to privacy and security. They use shared IP addresses, so when they say no one has any idea what you're doing when you're connected, they mean it.

Is all that is happening that connections on a specific port go to a certain computer and in that sense, while knowing the IP address isn't enough, knowing the port and IP would uniquely identify a computer? And IP packets contain the source/destination port so this really isn't saying much.

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NAT stores connections into a tuple in its NAT tables, for lack of typing an RFC like answer, works like this: Assume you and I are on the same network

You (10.1.2.3 internal) --> router/firewall 10.1.2.3:80 internal 1.2.3.4:1000 external
Me (10.1.2.10 internal) --> router/firewall 10.1.2.3:3000 internal 1.2.3.4:2000 external

When you make a connection anywhere, the device performing NAT stores info as a port (tuple) both inside and outside. When you leave, and come back, it matches this tuple. This is how both you and I, and others on the same network, can all visit Google simultaneously with the router/firewall knowing to send: "Pokemon" searches to you, and say "Cupcake" searches to me.

There is minor privacy behind NAT. While the average person will see one address, anyone with access to the firewall/router can pinpoint who did what. Further, on the public side of the equation, there have been discussions concerning the reality of privacy and NAT. See: "A Location-Privacy Threat Stemming from the Use of Shared Public IP Addresses"

Also see: "Analysis of Solution Candidates to Reveal a Host Identifier (HOST_ID) in Shared Address Deployments"

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  • So a shared IP address is always accomplished through NAT?
    – Celeritas
    Commented Jan 12, 2015 at 7:36

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