Don’t spend your time calculating the offset with your scientific calculator, just use CIDR Calculator.
CIDR Calculator v1.0 MacOSX Retail-CORE
designed out of a desire to streamline CIDR calculations by creating a simple, easy to use tool for performing CIDR network block math and network masks with complete confidence.
A cornerstone of worldwide IPv4 network communications is Classless Inter-Domain Routing, also known as CIDR. CIDR blocks help routers move packets based on a shorthand method of telling another system what IP ranges the devices can route. Now, if you’re looking at this tool, you most likely know more about CIDR than I can possibly cover here, so I’ll focus on the unique features of this Application:
CIDR Calculator is a simple to use tool that allows rapid deconstruction and understanding of any valid CIDR block notation.
For example. You’ve received a block of IP’s from your ISP. Maybe they are add-ons to your existing network. Perhaps you are deploying a completely new network. When you look at the provisioning worksheet and see the notation 220.127.116.11/25 you will want to know what IPs are part of that range. Instead of trying to do perform the 32-bit math in your head, or on a calculator, why not just use this handy tool? If you with a simple push of a button you’d have known:
* The true starting IP address of the block: 18.104.22.168
* The top, or end of the IP block: 22.214.171.124
* The decimal equivalent of both the starting and ending IP ranges. Useful in only a few situations, but when you need it, you really need it. (In this case they are 301713408 and 301713535 respectively).