If you want to access a name based website that you created e.g. in ISPConfig before the DNS records are setup, edit the hosts file on your workstation computer and add a line like this:
Replace the IP address with the IP of your server and www.mydomain.com with the domain name of the website.
The hosts file on Linux and MAC systems is /etc/hosts, on Windows systems the hosts file is:
The above trick also works if you want to run a website in your internal network (intranet) without having a DNS server available for local DNS resolving.