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Possible Duplicate:
How does Subnetting Work?

So if I add to my allowed IPs in security groups, does that mean that everyone with an IP from to can access the instance?

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marked as duplicate by Chris S Aug 16 '12 at 15:00

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

That's CIDR notation: and there are CIDR calculators, e.g., – cjc Aug 14 '12 at 13:12
The /32 means that all 32 bits need to match. – David Schwartz Aug 14 '12 at 13:38

If you mean the well-known IP/subnet notation, then this specifies a single IP address because of the subnet mask /32. If you want to have the range from 0 to 255 in the last octet, you need to specify /24 as subnet mask.

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