system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Knowing that IP Addresses generally aren't static, how does online activity get tracked over time?

Furthermore;

* If IP addresses change frequently, why do hackers use proxies?
* How far back in time can online activity be tracked?
* How can one protect himself more efficiently when making online transactions?
* Can/should a Mac Address be hidden/disguised?
* Is there a way to see, by number configurations(or otherwise), if an IP is fishy?

Cheers!

EDIT: Hey thanks for the responses everyone! I learned a good bunch with this thread. You guys rock!
gpt
* Because ISP:s log who's used what IP when
* Depends on how old logs you can find
* For bank transactions: Make sure you're on the right site, check it's using SSL. For mostly anything else: Using anonymization (Tor or proxies/VPN), NoScript to block Javascript, and other techniques. Also, by minimizing the info you share.
* MAC addresses are only visible inside a network.
* No.

Edit: To answer the title question: "Cookies". Sites ask your browser to store unique ID strings that the site can ask your browser about the next time your visit them. This include ad networks.