ip address that isnt valid anymore

To clarify a few things... just because your ISP assigned IP address has changed does not mean that your previously assigned one is invalid. It's not assigned to you anymore, but, it probably will be quickly assigned to someone else from your ISP. And not that it is too important, but, the reassignment will likely be to someone in a close geographic area to you. That's usually how ISPs group and assign ranges of IP addresses - to a physical area.

Now, what that means for your postings on forums is that you end up with a history of many used IP addresses, all specific to which one you were using at the time of each post. But, there is very little more than that. Reverse lookups can be done on all your old IP addresses, and depending upon how your ISP sets up their name resolution, there may be a small amount of general information available about where the IP address is assigned.

A lot of cable and phone company based IP addresses will reverse to something that includes a city or regional reference. (Some US ISPs also use airport abbrevs as a geographic reference.) Even if there isn't a reverse lookup for the specific IP address, often a WhoIS reference may still provide a regional location for a range of IP addresses. But, that's really all.

The average person isn't going to know too much about you just by knowing an IP address that you've used. Your name for example is not going to be publicly assigned to your address. But, your ISP is definitely known since all IP addresses belong to some company, and that can always be looked up. The location, by either region or city, is known maybe 50% of the time, based on ISPs including that information in their assignment of IP address ranges.

Finally, as the posts above note, this is all that can be determined by the average forum owner. Without access to the ISP's internal information of who was assigned what IP address when, nothing more can be determined. So, short of a law enforcement inquiry, the most that anyone who can see your IP address will know about you is that you use ISP: "XYZ Cable Company" and maybe a city or regional location.

 

No, no... not simultaneously assigned... It's one after another.

IP addresses are at a premium on the Internet. When an ISP changes IP addresses regularly, let's say because they assign simple dynamic IP addresses for dialup or ADSL, those addresses don't stop being used when they change. They get reassigned from one customer to another.

Because of the "time". You have a history of many IP addresses as you use the Internet over time. When yours changes to a new address, you get logged as you access websites with that address. But, there is the time component involved. If you use 1.1.1.1 today and 1.1.1.2 tomorrow, it is clear you have changed. If someone else uses 1.1.1.1 on the same website, sure, the webmaster might be suspicious that it is still you... But, the Internet is a huge place! Do you really think, using this website as an example, that your neighbor 5 doors down from you is going to show up at Wilders Security using the same IP address that you used the day before? What are the odds? There are countless millions of websites on the Internet. What are the odds you and they ended up here a day a part using the same recycled IP address? Billions to one?!

As a difference, certainly if you use a proxy server, like TOR or Xerobank, then yes, you are using a shared IP address. And yes, if two Xerobank customers using the same config access Wilders Security a few hours a part, they could use the same IP address (exit node) and look like the same person. But again, what are the odds? Thousands or a million Xerobank customers and you and another both end up here at Wilders Security within a few hours of each other? Very unlikely.