I have ever guess that IP-3:80 is not listened by nginx.But when I run "netstat -antup" in command line of shell,the result following tell me that I guess wrong.
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 IP-3:80 0.0.0.0:* LISTEN 7346/nginx
tcp 0 0 IP-2:80 0.0.0.0:* LISTEN 7346/nginx
tcp 0 0 IP-1:80 0.0.0.0:* LISTEN 7346/nginx

OMG,It's a very strange problem.