An autoblock is an automatic block of an IP address. Autoblocks are the result of an attempt to edit Wikipedia from an IP address recently used by a blocked user.

Each time a user edits Wikipedia, the IP address from which the edit was made is logged (this log is accessible only by a very small number of trusted users, called checkusers). If a blocked user attempts to edit, the IP from which they are editing is "autoblocked", to prevent their switching to editing anonymously or under a sockpuppet username.

It is possible for innocent users who have done nothing blockworthy to be blocked from editing by an autoblock. If this happens to you, simply request an unblock and an administrator will review the situation.

Users who find themselves autoblocked accidentally may request an unblock by one or more of the following methods:

1. Posting the {{unblock-auto}} template on their talkpage to obtain the attention of an administrator (see {{autoblock}} for further instructions)
2. Contacting the administrator who issued the initial block and requesting an unblock, by sending them an email with the "Email this user" option at that admin's userpage
3. Going to the #wikipedia-en-unblock IRC channel on Libera Chat
4. Submitting a request to the Unblock Ticket Request System

If a user is autoblocked, an admin can clear the autoblock by:

• Reviewing the list at Special:BlockList, and searching for the user's account name. This is a case-sensitive search.
• Identifying the #xxxxx number that is associated. If searching on this number, be sure to include the "#"
• Unblocking the #xxxxx by clicking the "unblock" link

Note that once the autoblock is cleared, the user's account name will no longer appear in the list.

If the user has posted the blocked IP on-wiki as part of the request to be unblocked, consider whether there is a need to remove this information for privacy/security reasons, using revision deletion.

When a user is autoblocked, they see the following message:

Sometimes the term Autoblock disabled will show up in a block message. This means that when the user was blocked, that only their username was blocked, but other users on the same IP are still free to edit.

The default setting for a block is to have Autoblock enabled, but it generally does not say this explicitly in a block message. To disable the default autoblocking of an account, admins must manually uncheck the box that says "Autoblock any IP addresses used".

An example of autoblock in action:

1. Bob's account "User:Bob" is blocked for a week with autoblock enabled. Bob was using IP address 192.0.0.1.
2. Bob stays on IP address 192.0.0.1 and attempts to edit both logged out and from their unblocked account "User:Bobby". They can not do so as IP 192.0.0.1 is autoblocked for 24 hours.
3. Bob moves to IP address 192.0.0.2 and attempts to edit both logged out and from their unblocked account "User:Bobby". They can not do so as IP 192.0.0.2 is autoblocked for 24 hours.
4. This will continue until the "User:Bob" account is unblocked after a week.

When an autoblock occurs, users may be autoblocked as the result of a block on another user, who was probably using the same IP address. So a different user may end up blocked, even though they have personally done nothing wrong. This is referred to as "collateral damage". Example:

1. User:Alice, an administrator, blocks User:Bob for 24 hours. Unknown to Alice, Bob uses an ISP that uses dynamic IP addresses (such as at home, at university, at a library, at a company, etc.), and an autoblock was enabled at the time of the block.
2. User:Charlie, who also uses the same ISP, and is currently assigned the IP address last used by Bob, signs on to Wikipedia.
3. Charlie receives a "You have been blocked" message, doesn't understand what an autoblock is, and angrily demands to know why admin Alice has blocked him.

It is important for users to understand that administrators do not set autoblocks; once they have blocked a user with autoblocking enabled, autoblocks are set by the MediaWiki software. Autoblocks do not appear in administrators' block logs, and the administrators are not notified of them. This is a necessary consequence of Wikipedia's privacy policy, to keep logged-in users' IP addresses private. So while the IP address responsible for each edit is recorded by the MediaWiki software, this cannot be accessed, even by administrators and even when the user is blocked.

The only circumstances in which a user may be associated with an IP address, are certain policy violations detailed by the checkuser policy; even then, access to the Checkuser tool is granted only to a small group of trusted users, not administrators in general.

When a user is blocked with autoblocking enabled, a cookie will be set on the blocked user's browser for tracking purposes. This means that the user will still be blocked even after logging out and moving to a new IP address. When the cookie is set, and a user tries to edit a page, MediaWiki will load the original block specified in the cookie and prevent the user from making the edit. This does not, however, create a new autoblock for the new IP address. That only happens if the user logs into the original blocked account. This form of tracking is dependent on the user's browser retaining the cookie, and so will not work for all blocked users in all situations. The feature is intended to provide a small extra level of protection against blocks being circumvented.

When a block is issued, autoblocking is usually turned on by default, except for common dynamic IP ranges, such as those used by many residential ISPs. A list of such automatically exempt IP ranges can be found at MediaWiki:Autoblock whitelist.

Administrators can disable autoblocking at the time of blocking a user, by unchecking the checkbox. Once an "enabled" block is placed, it can also be fixed by modifying the user's block, but this should not be done unless absolutely necessary.

There is an internal autoblock expiry time variable, which is set to 24 hours, meaning that autoblocks only last for 24 hours. However, in the case of dynamic IP pools (such as those used by many residential ISPs), this may affect hundreds of users before the block expires. So in the case of an indefinite block, autoblocks may continue to be set by the software, weeks or months after the initial block has been set. Older indefinite blocks, dating from before the autoblock exemption whitelist and the option to disable when blocking, may also trigger autoblocks.

Since 2012 (Mediawiki v. 1.20.0), manually lifting the original account block automatically lifts every auto block that was created as a consequence of that block. (If the original block merely expires automatically, any dependent autoblocks are not immediately lifted.)

When IP addresses are autoblocked, they appear in Special:Ipblocklist (but not in the admin's block log) with a special mask that prevents the IP from being seen. Autoblocks register on Special:Ipblocklist and in the banner available to the blocked user, with the name of the admin that set the original block. However, the admin is not notified that an autoblock has been placed.

• It is helpful for all involved to remain patient and remember that it is the software, not the administrator who is unfortunate enough to have their name appear on the block, that is responsible for the autoblock.
• Equally important is for admins to check Special:Ipblocklist regularly, and unblock all autoblocks from a particular user if more than two autoblocks are set in rapid succession.
• If more than two IPs are autoblocked within seconds/minutes of each other, it is a good indication that it is a dynamic IP pool and the blocks are collateral damage.

• Wikipedia:New admin school/Blocking