Subnet Calc is a subnet / ip calculator designed to help network administrators who need to remember details about the various subnets which they administer. You can select an IP and a subnet in CIDR notation and Subnet Calc will tell you all the details about the subnet, including its class and even whether or not the IP is in the loopback / private ranges.

There is also a PRO version called Subnet Calc Pro. Be sure to check it out for more advanced features such as an organiser and the ability to find a subnet based upon a given set of IPs.