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What kind of thing is legal to do once but not 65,535 times?

Google for "scan ports" and you'll discover plenty of people offering this service already.

"Sometimes, if a computer system is affected too much by a port scan, one can argue that the port scan was, in fact, a denial-of-service (DoS) attack, which is usually an offense. "

http://www.sans.org/security-resources/idfaq/port_scanning_l...

Rate limit target IP subnets. 0.1 second timeout per port per IP: ten ports on one IP delays a second, one port on everything in a /24 delays 25.5 seconds. It's useful without being abusive.

You could require verification for >5 ports.

>What kind of thing is legal to do once but not 65,535 times? //

Knock on someone's door? Once is fine, more than about 20 times [in a day] is likely to be seen as a nuisance. You'd have some sort of exclusion order long before you get to 2^16 times.

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