Syntax:  limit_conn zone number;
Default:  —
Context:  http, server, location

limit_conn_zone $binary_remote_addr zone=addr:10m;

server {
  location /download/ {
    limit_conn addr 1;
  }

http {

... ...

  limit_conn_zone $ip_segment zone=network_segment:10m;

  server {
      ... ...
      set $ip_segment $remote_addr;

      if ( $ip_segment ~ ^(\d+)\.(\d+)\.(\d+)\.(\d+) ) {
        set $ip_segment $1.$2.$3;
      }

      location /download/ {
        limit_conn network_segment 10; 
        #limit_rate 200k;
      }

  }
}