I should add that a good place to learn about why you're now using /16 instead of /24 would be to learn about Subnetting [en.wikipedia.org]. It will save you some grief later, expecially if you're messing with firewall rules. :)
I've been in the Internet industry for quite a while and have never heard of an ISP suing anyone because they were blocked by their firewall rules unless there was some sort of pre-existing contractual relationship where they had agreed to pass traffic for them.
Obviously they wouldn't have a legal leg to stand on, somehow trying to force someone into allowing their packets on the other persons server or network.