How to update xcode via app store?

I just update xcode from 4.2 to 4.2.1 via App Store. However, after downloaded the latest version of xcode, there is nothing happens!
I searched the internet, and finally found that I must run /Applications/install xcode.app.... it means App Store only download the install package, and place it into /Applications, but doesn't run it for you.

You can click "install xcode" from launchpad, and follow the instructions.

For windows users, we all know that we can edit c:\windows\system32\drivers\etc\hosts to specify an IP address to a host name. But where is the "hosts" in Mac?

I searched the internet, and find out that this file is on: /private/etc/ so the file is:

/private/etc/hosts

And the format is the same as other systems.