Say you’ve got your webserver listening on port 80 and all requests from your router for webpages point to it (server0).
Now say that server0 handles Name based Virtual Hosts. So the one Apache web server is hosting a whole bunch of domains: example.com example2.com example3.com etc.
What do you do if you want to run a second server (server1)on the same IP address?
Well you could have another server there just to load balance or you could get the router to do it (if it was smart enough).
But another way is to setup server0 to proxy connections of your choice over to server1.
<VirtualHost *:80> ProxyPreserveHost On ProxyPass / http://192.168.1.9 ProxyPassReverse / http://192.168.1.9 ServerName example.com </VirtualHost>
Server1 has Name based Virtual Hosts as well and doesn’t need any special configuration.