IP's aren't encrypted, they are in a different number base. You'll find them as Hexadecimal without the .'s
There is a function someplace (either in the phpBB code or in the database server) which will do it for you.
You can do it by hand too. Each portion of an IP address is two hex digits. You can use the windows calculator or some other calculator (or if you are really good in hex, your head) to do it two hex digits at a time.