IP blocking prevents the connection between a server/website and certain IP addresses or ranges of addresses. IP blocking effectively bans undesired connections from those computers to a website, mail server, or other Internet server.
Unix-like operating systems commonly implement IP blocking using tcp wrappers, configured by host access control files[1] /etc/hosts.deny and /etc/hosts.allow.
IP banning is commonly used to protect against brute force attacks. Both companies and schools offering remote user access use Linux programs such as DenyHosts or Fail2ban for protection from unauthorized access while allowing permitted remote access. This is also useful for people who want to remotely access their computers. It is also used for censorship.
On an Internet forum or Web site an IP ban is often used to prevent a disruptive member from access, though a warning and/or account ban may be used first. Dynamic allocation of IP addresses can complicate incoming IP blocking, rendering it difficult to block a specific user without blocking a larger number of IP addresses, thereby risking collateral damage caused by ISPs sharing IP addresses of multiple internet users.
Cyber bullies have developed a novel use for IP blocking that works to their advantage. They send an unthreatening email to an unsuspecting recipient. When the recipient responds, the cyber bully obtains and can then forge their IP address to send out emails, with the intent of getting the IP address banned and blocked. The recipient's subsequent emails are 'blocked' and returned 'undelivered'.
IP banning is also used to limit the syndication of content to a specific region. To achieve this IP-addresses are mapped to the countries they have been assigned to. This has been used to devastating effect most recently to target Nigerian IPs due to the perception that all business emanating from the country is fraudulent. Thus making it extremely difficult for legitimate businesses based in the country to interact with their counterparts in the rest of the world. To make purchases abroad, Nigerians rely on proxy companies to mediate transactions.
Proxy servers can be used to bypass an IP ban unless the site being accessed has an effective anti-proxy script.