Another little chart you can use to help you learn the relationships between binary and decimal values, subnet masks and prefix lengths and your powers of 2.  The chart allows you to practise by filling in the appropriate subnet mask or prefix length and then read the other value from the chart.  The decimal bit value row is also the same number as the "block size" for newly created subnets.  Meaning the subnets will all be multiples of this number.  Updated 5/28/2011 to include notes and links referencing the /31 prefix length.  I hope you enjoy and let me know if you have any questions or comments.  Thank you.