This is an explanation as to the different types of IPs.
There are 3 types of IPs: Static, dynamic and shared.
Static means that whenever a user is online, they are assigned the exact same IP from their ISP. This is typical for connections that are always on, like LAN, DSL, etc. You can usually tell if it's a static IP if the IP is posting to the same article(s) over and over again over a 3-4 day period or longer.
Dynamic IPs are what most dial up users (including AOL) have. It means that everytime they are online, they receive a different IP address. Dynamic IPs are usually easy to spot because they post to vastly different articles (like ESPN one time and garden the next). We usually don't block dynamic IPs for long because it could cause collateral damage. I.e. as soon as that user goes offline, the IP is assigned to someone else. So a long block could block people that have nothing to do with the vandal.
Then you have shared IPs. They are mostly at schools. It means that several people share one IP. If you block them, you are blocking every person who uses that IP address, which is why again, we keep it short.