Internet Protocol (IP) addresses link a single computer or a small network of computers connected to the Internet to a location. IP addresses can be considered bad for a variety of reasons. Sometimes this results from poor net etiquette by the owner of the address, and other times "bad" simply refers to the wrong address.
Blacklist
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The term "Bad IP address" typically refers to malicious activity by the owner of the address. Internet users who spam other people's email addresses or send viruses and other malware, as well as unethical websites that attack, phish for personal information and drop fake anti-virus software all fall under this category. Several websites compile lists of these types of users and malicious websites. Then, online email services and true antivirus software use the list of IP addresses to form a blacklist that protects their users.
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Malware
Websites that drop malware onto visitor's computers often get added to bad IP lists. A November 2010 survey of worldwide bad IP addresses from malware dispersion indicated that over half came from the United States. This should not come as a surprise as the U.S. has the most IP addresses in general. However, runner-up countries like China and Romania draw concern because their ratios of bad to total IPs are especially alarming. Some servers prefer to block entire countries rather than try to sift through the good guys and bad guys.
Spam
The industry standard definition of electronic spam is unsolicited bulk email. To be officially considered spam, an email has to be both verifiably unwarranted and sent in bulk to a large number of people. Someone who sends spam risks having his IP address added to a blacklist, preventing many recipients from ever getting his message. This happens after a number of recipients of the spam click "Report Spam" or similar links in their inbox.
Bad Domain Name
An IP address is a set of numbers separated by decimals. Although it can represent the general location of a computer, IP addresses can also represent individual web sites. In 1984 the domain name system (DNS) was introduced to make website names easier to remember. Either the IP address or its respective domain name may be typed into the address bar to reach a site. If a website delivers an unexpected error message, then check to make sure you typed it correctly. You may have the wrong domain name, or the site may have been discontinued or moved.