What are IP Address Blacklists and What Consequences May Arise if an IP Address Gets Blacklisted?
Imagine yourself for a moment as a mail carrier delivering mail to the apartments in your neighborhood. Out of a thousand apartments, five are inhabited by aggressive drug addicts who regularly attack mail carriers; after a couple of unpleasant encounters there, you start to avoid those apartments.
You share this information with your fellow mail carriers, as well as with other services responsible for visiting the apartments. For example, if the specialists responsible for checking meters go to the apartments without your blacklist, they will surely remember their encounter with the drug addicts for a long time. But if they use your blacklist, they will avoid all the "dangerous" apartments.
The situation is similar with IP addresses: there are companies that track IP addresses involved in DDoS attacks, fraudulent activities, and spam distribution, and add these addresses to their blacklists. Then other companies, mainly email services, internet providers, payment systems, banks, and online stores, purchase checks of their visitors' IP addresses against these databases.
For example, when a customer of an online store enters their credit card information to pay for a product, the store checks whether the user is on any blacklists. If the IP address is on the blacklist, they are highly likely to be denied or sent for additional verification. This is done due to fraudsters using stolen credit cards, which become a problem for the stores.
I want to note that in the case of user verification and assessment by modern anti-fraud systems, things are a bit more complicated than a simple check of the IP address against blacklists, and the presence of an IP address on a blacklist is just one of the indicators that form the final user assessment. Therefore, getting an IP address blacklisted is undoubtedly bad, but "not critical."
Particular inconvenience for users of "dirty" IP addresses may come from the constant introduction of CAPTCHAs, for example, when using search engines like Google and Yandex, or undergoing checks from anti-DDoS systems like CloudFlare.
Users of VPNs suffer from this particularly often. The IP addresses of public VPN services regularly end up on blacklists because cybercriminals often resort to the services of VPNs. If the IP address of a VPN server gets blacklisted, all users connected to that VPN server will encounter problems.