Add the desired ports to the httpd.conf (or ports.conf file if you have one).
# Uncomment the following if you will be using SSL
# Listen 443
# Add any additional ports here.
# Listen ??
You can also specify a listen on only one ip/port:
Next you need to set up virtual hosts on the desired ports. The easiest way to do this is to create a custom .conf file for the host with all of the settings in it and include it at the end of your apache2.conf or httpd.conf.
It is actually best to use names when you can; if you choose to do that you can do name based hosting. I wrote a post
on apache2 with virtual hosting if you need more resources.