Nginx: how to set default server

Sometimes it's conveniently to set up default server, that gonna try to serve request by default. Originally, if your server got a request by IP address (for example) and can't get any requested hostname, it takes first server description from your config and tries to serve request with this server.
You can change this behavior. It's really easy. Just use default_server parameter, like this.
server {
    listen      80 default_server;