Well, you mean not very usefull for you
The only way to obtain the full content ID, is to download the full pkg and watch it in hex. But who wants to download GBs of pkgs only for knowing the right ID?
This small app loads only the few bytes from the pkg which contains the content ID.
This is usefull for people who are working with psnlinks, rif and rap
ask google for the right contentID and the rap