Manual:$wgPutIPinRC

Prescribes whether MediaWiki should log the IP addresses of its editors in its database's recentchanges table. Although there is no interface in the standard MediaWiki package for retreiving this information, it can be accessed using extensions; a database administrator can also run an SQL query to retrieve the IP addresses.

Current versions of the CheckUser extension do not rely on this setting; CheckUser stores its data in a separate  table.