Subdomain an IP address

[mbrosius]

Here is my problem:
I would like to have subdomains on an IP address. I do not have a registered domain so this is what I want it to look like:

http://customers.IP_ADDRESS

Here is the process:
I will send an e-mail to a customer asking them to click a link called

http://customers.EXTERNAL_IP_ADDRESS

I have a firewall which will redirect all HTTP traffic to an internal address. Therefore the customer will get the information from

http://customers.INTERNAL_IP_ADDRESS

Here is part of my httpd.conf file:
Port 80
ServerName 192.168.100.202
NameVirtualHost *
<VirtualHost *>
DocumentRoot /var/

http://www/html

ServerName 192.168.100.202
</VirtualHost>

<VirtualHost *>
DocumentRoot /var/

http://www/html/customers

ServerName customers.192.168.100.202
</VirtualHost>


Do I need to do anything with DNS? If so, what?