Wikipedia:Restrictions on Anonymous Editing from Shared IPs

This policy aims to restrict anonymous editors from shared internet protocol addresses (shared IPs) from modifying mainspace articles. This includes many mainstream internet service providers (ISPs) such as America Online and Verizon Broadband, as well as many schools, large businesses, and congressional offices. Shared IP users may still edit talk pages, user talk pages, and any page not within the main article namespace. If a user from a shared IP wishes to contribute to the main article namespace, he or she may register for an account to do so.

To Protect the Ability of Any User to Edit Without Registering
Currently, when a shared IP address is blocked, it prevents potentially thousands of users from editing Wikipedia, whether registered or not. This policy would serve as a compromise, whereby anonymous editors from shared IP addresses may not edit within the main article namespace, yet the likelihood of a shared IP being blocked would be drastically reduced, if not eliminated altogether. Thus, with this policy in place, more users would be granted their right to edit without the risk of being blocked for the actions of one user.

To Ensure that Messages are Delivered to the Intended User
Currently anonymous users from shared IPs frequently receive vandalism warnings and messages that have nothing to do with their own contributions. This frequently upsets and confuses many new users and often discourages many potentially excellent contributors from further editing Wikipedia. With this policy in place, anonymous users would only receive messages intended for them, and as such others do not need to fear biting new users when posting vandalism warnings.

To Reduce the Prevalence of Vandalism
As users from shared IPs can quickly switch their IP address, by logging off and logging back in, the ability to block vandals operating from shared IPs is highly limited. With this policy in place, all users making edits to the article namespace must either be registered or operating from a static IP address; thus, administrators can successfully block abusive users from editing and thereby reduce a vandal's ability to perpetuate vandalism further.