I have a site hosted at Google App Engine. Its current address is in the format like
xxx.appspot.com. Now I want someone to have access to it by typing some IP along with sub-directory, like
xxx.xxx.xxx.xxx/dir1/dir2/dir3. How can I do that?
This would be near-impossible to accomplish. It is easy to set up a GAE app on a custom domain, but it seems the OP has some more specific requirements.
xxx.appspot.com domain can certainly be translated to an IP address, but GAE's servers will not be able to respond to requests by IP address. It has to know which app you are trying to access, and cannot do so without seeing the hostname. Also, Google might change the IP address associated with your hostname at any time, so you cannot depend on it.
Additionally, making it respond to a sub-path is not easy. If it is at all possible, it would involve altering your app, so that it basically ignores requests to anything but the desired sub-path.
However, I suspect you want to do this because you want to run other (non-GAE?) services on the same IP (that you own), while having your GAE app "installed" on the specified path. This is generally not possible. You might be able to work around it by setting up a web proxy on the desired path, which forwards all requests to the GAE app.