Apache Bind problems - Will 'Listen 80' but not to specific IPs, how to bind to specific IPs?

Hello!
I am trying to bind my Apache2 to specific IP addresses. Currently in the apache conf I have:

Listen 80
and then in my virtual hosts config I have:

NameVirtualHost 80.xx.xx.15:80
NameVirtualHost 80.xx.xx.17:80

<VirtualHost 80.xx.xx.15:80>


When I change the Listen part to :

Listen 80.xx.xx.15:80
Listen 80.xx.xx.17:80


Shutdown the server, check that nothing is running on port 80, and restart I get the error message:

(98)Address already in use: make_sock: could not bind to address 80.xx.xx.15:80 no listening sockets

and Apache exits. I have no other Listen's elsewhere in the code (I do have a ServerName outside of the Virtualhost, if that makes a difference?). I change it back to Listen 80 and it works again no problem! I'm limited to experiment too much, as it's a live server.

What could be the problem? Please help!