how to configure multiple virtual server on nginx?

[debugger87]

Hi,all
Pardon please,I'm not an English native speaker.So,I may make mistake in
grammar.
I have 4 IP address on my ubuntu server computer.For the convenience to
ditinguish them ,I name them as IP-1,IP-2,IP-3 and IP-4.
Now,I configure 3 virtual servers on nginx.The key content of
configuration is following.

#upstream
upstream my_upstream{
		server IP-3:8080;
		server IP-4:8080;
}   

#the first server
server {
    	listen IP-1:80;
	location / {
			index index.php;
			root /usr/share/nginx/www;
	}
	location ~.*\.(php|php5)$ {
			root /usr/share/nginx/www;
			fastcgi_pass 127.0.0.1:9000;
			fastcgi_index index.php;
			include fastcgi_params;
	}
}

#the second server
server {
        listen IP-2:80;
        location / {       
		index index.php;
        	root /usr/share/nginx/www;
        }
        location ~.*\.(php|php5)$ {
        	root /usr/share/nginx/www;
        	fastcgi_pass 127.0.0.1:9000;
        	fastcgi_index index.php;
        	include fastcgi_params;
        }
}

#the third server
server {
    	listen IP-3:80;
		location / {
			proxy_pass http://my_upstream;
		}
		location ~.*\.(php|php5)$ {
			root /usr/share/nginx/www;
			fastcgi_pass 127.0.0.1:9000;
			fastcgi_index index.php;
			include fastcgi_params;
		}
}
########################################################

In my opinion,those 3 virtual servers can wholely work right.But I met a
very surprising problem:the 1st and 2nd server can work
right,nevertheless，the third virtual server can not access.I look over
nginx.conf,and never find out any configuration mistake.Then,I change
the 3rd server's LISTEN IP address IP-3 to IP-2,and change the 2nd
server's LISTEN IP address IP-2 to IP-3.Oh my god,the 2nd server can not
access,but the 3rd can.I just exchange their LISTEN IP address.If you
have  understood my problem description,would you please give me some
good suggestion?
Thanks!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,212966,212966#msg-212966