Re^2: Need to calculate IP address

s/\d+$/ $& ^ 1 /er

A neat solution, of course, but unfortunately it doesn't meet the OP's specs: .5 becomes .4 and .6 becomes .7.

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Re^3: Need to calculate IP address by tybalt89 (Monsignor) on Dec 28, 2017 at 19:44 UTC
I think the example is wrong. What should a 0 become? Or a 9?	[reply]
Re^4: Need to calculate IP address by ddrew78 (Beadle) on Dec 28, 2017 at 20:01 UTC
It will never be a 0. If the IP ends in .9, it needs to become .10. .10 would become .9. .29 would become .30, etc.	[reply]