How To Make Ubuntu Apache2 Listen on another port

red@ubuntu:/etc/apache2$ sudo cp ports.conf ports.conf_old
red@ubuntu:/etc/apache2$ vim ports.conf
red@ubuntu:/etc/apache2$ sudo vim ports.conf
red@ubuntu:/etc/apache2$ diff ports.conf ports.conf_old
8c8
< NameVirtualHost *:*
---
> NameVirtualHost *:80
10d9
< Listen 81