how can GAE provide different APPs via different URLs using same IP address?

for example, when I ping mark.appspot.com and john.appspot.com, it returns 开发者_C百科the same IP, but these 2 URLs are different APPs.

since these APPs have the same IP, the GET HTTP package will be same. My question is: how can the server return different contents from the same HTTP package?

This is known as "virtual hosting" read about it at http://en.wikipedia.org/wiki/Virtual_hosting. This technique is used by all shared web hosting providers.