For those who are studying CCNA, well I would recommend the following sites and tools.
This is a web based subnet calculator and is written in php. Use it to check your manual calculations.
Under the download section of the webpage, you will get to download a Subnet Calculator and a TFTP server for your use. The downloads are of course free.
Qod.us is short form for Question Of The Day. What this site does it that it e-mails you a question on a daily basis for you to attempt. The benefit is that you get to practice daily and best of all, it is free but requires registration.
If you have any other useful links that will be of good help, feel free to post in on the comments of the post.
Benny Ong XXX