In using a combination of jbothe's code below and some of the "$mask = 0xFFFFFFFF << (32 - $bits)" type code, I ran into an error with some later calculations on a 64 bit machine.
Keep in mind that when you're analyzing numbers meant to be treated as 32 bits wide (such as IP addresses), you may want to truncate them. Without relying on other libraries, it was simple enough to follow any calculations that may end with different results on a 64 bit machine with " & 0xFFFFFFFF"
Though in many cases, it seems like it might be simpler to just use "~0 << ..." for initial shifting to create the network mask instead of "0xFFFFFFFF << ...". I don't know that it guarantees further operations to work as expected though.
