Blacklisting in the context of Internet use pertains to the act of placing an IP or email address into a list of spammers, hackers, and the like in order to block them from posting or sending any more of their spam-related messages. Meanwhile, the blacklist refers to a list of IP addresses and email addresses that are believed to have come from suspected or known spammers or hackers. Both companies and individuals can use the blacklisting method to block online content believed by the user to be harmful or bothersome to his everyday computing experience.
Quite a lot of email programs nowadays contain blacklist-based filtering features as well. At any rate, an online blacklist is also known as a blackhole list, and it's usually composed of anything that can be blocked off a given network—that is, ISP addresses, email addresses, certain keywords, certain texts, certain user behaviors, and even the mere act of spamming can serve as indicators for blacklisting.However, most blacklists only really need to contain the ISPs of the most notorious and infamous of spammers in order to be effective, such as the RBL (a blacklist that's maintained by MAPS LLC).
Meanwhile, in computing terms, a blacklist (otherwise known as a block list or a list of denied accesses) is a basic access control mechanism that allows everybody entry into a given system save for people who are part of the blacklist. The opposite of a computing blacklist is naturally a whitelist, which works in reverse (that is, the mechanism only allows the ones who are part of the whitelist to gain access, while all other outsiders are blocked). In turn, a greylist serves as a middle ground of sorts between the blacklist and the whitelist, such that it contains a list of blocked people who are either temporarily allowed or blocked access.
A company may maintain a blacklist of websites or programs in its computer system or the individual workstations of its staff. The indexed items are banned from use or access and everything else would be enabled. For instance, a university can blacklist instant messengers from Yahoo or AOL, the freeware mIRC chat program, the KaZaA P2P program, and ICQ. Also, social networking sites like Facebook, YouTube, and MySpace can also be blocked. Meanwhile, the rest of the Internet services would still be allowed for use.