how to write a java program that will take an ip address from client
as input(4 octet number separated with "dot")and then tell the client
what class that ip address is, and the netid, and host id.

basically in java there are libraries where you can include to do
network procedures.

http://planet-source-code.com
you can find source code in java doing something like you want.

Search criteria:
Java ip class  

and i got
Command-line based program that will lookup a given hostname and find
its corresponding IP address and also determine the network class.

i hope it helps.
Phossil