Cassette Lockring or Freewheel Tool - FR-5.2 - from Park Tool USA

Also fits Shimano disc brake lockrings.

The FR-5.2 has 12 splines that are approximately 23.4mm across.

Note: Always use a quick release skewer to hold the tool in place and snug to cogs when loosening.

For solid axles, use the axle nut

Note: You will also need a chain whip tool to hold the cog in place.

Weight: 61.6g (2.2oz) 

Park Tool Reference: FR-5