Manual:$wgBlockCIDRLimit

Details
Maximum CIDR range when blocking an IP range.

Default values
Before MediaWiki 1.20wmf4:

MediaWiki 1.20wmf4 and later: (See 11259 and 11260)

This setting does not control ext>Special:MyLanguage/Extension:GlobalBlocking|Extension:GlobalBlocking.

Regardless of this setting, GlobalBlocking can block IPv4 ranges up to /16 only, but has no limit on IPv6 range sizes ().