The Safari version of the CCENT/CCNA 640-822 Official Cert Guide, Third Edition contains more than an hour of video training from the author on the subject of subnetting. The videos are divided into eight lessons:

Part I: Finding the Subnet Number
Video 1: Example 1
Video 2: Example 2
Video 3: Example 3
Part II: Finding the Broadcast and Range of Addresses
Video 4: Example 1
Video 5: Example 2
Video 6: Example 3
Part III: Finding all Subnets
Video 7: With Less Than Eight Subnet Bits
Video 8: With More than Eight Subnet Bits