It seems that local pc cannot access website external ip address via router port forwarding.
What you can do is add ServerAlias to httpd.conf like this:
Code:
<VirtualHost *:80>
ServerName www.mydomain.com
ServerAlias www1.mydomain.com
.....
Assuming that your webserver ip address is 192.168.1.2, add this line to /etc/hosts of Linux pc and/or C:\WINDOWS\system32\etc\hosts
Code:
192.168.1.2 www1.mydomain.com
Edit: Should be C:\WINDOWS\system32\drivers\etc\hosts