1、反向代理，不需要服务器池，直接代理某台服务器

location / {
        proxy_pass      http://192.168.18.201;
        proxy_set_header Host  $host;
                  proxy_set_header X-Forwarded-For $remote_addr;
}
 proxy_set_header Host  $host;                                      nginx请求地址池时带着客户端请求的主机头
proxy_set_header X-Forwarded-For $remote_addr;      负载均衡器开启ip转发，将客户端ip映射到web服务器

2、负载均衡，后端有服务器池upstream

upstream webservers {
      server 192.168.18.201 weight=1;
      server 192.168.18.202 weight=1;

location / {
              proxy_pass      http://webservers;
              proxy_set_header Host  $host;
                               proxy_set_header X-Forwarded-For $remote_addr;
      }