Last week I had been refactoring some code related to IP Addresses in our internal software at Acceleration.net. In our old code, I came across a pretty speedy ip-address printer that was faster than a naive approach by a good margin. A few days latter Stas Boukarev (stassats) happened to be discussing optimizing this function in #lisp. I sent him our slightly optimized version and after pasting back and forth, by the end of the day he had a very rapid function.
I added a pretty speedy IP address parser and the result of this work is cl-cidr-notation (https://github.com/AccelerationNet/cl-cidr-notation). This provides fast portable functions for reading ip-addresses and cidr blocks from strings into ints and for writing those ints back into dotted-quad notation (188.8.131.52), and cidr blocks into their standard notation (0.0.0.0/30). It can also efficiently print range strings (0.1.2.3-184.108.40.206).