Subdomain configuration problem

Hi,

I’m beginner in nginx, and i have searched google and mailing list but
no
luck.

my nginx.conf:

http{
server{
listen 80;
server_name example.com www.example.com;
location / {
proxy_pass http://127.0.0.1:aaaa/;
}
}
server{
listen 80;
server_name subdomain.example.com;
location / {
proxy_pass http://127.0.0.1:bbbb/;
}
}
#rest of the default config like acces log, etc
}

Problem is after adding server block for subdomain, both request (
example.com, subdomain.example.com) loading only example.com in browser.
If i place subdomain server block listing as first one, both request (
example.com, subdomain.example.com) loading subdomain.example.com in
browser.

Please suggest me what could be the problem.

Thanks in Advance!!