If you expose your Web Active Directory web application to the outside world on the Internet then you will likely want to secure the communication channel using SSL. One best practice, making it simpler for end-users, when using SSL is to use the default port of 443 and assign different host headers (subdomains typically) to different sites.
Setting up host headers with SSL in IIS 7 and IIS 7.5 can be challenging. The articles below help give insight into the alternatives available for SSL on IIS 7 and IIS 7.5, including how to set up host headers using Internet Information Services Manager on Windows Server 2008 and 2008 R2.
Use these articles to help guide your approach and remember that there are no wrong answers when setting up a publicly-accessible URL…you just want to make it easy and memorable for your users. You can even get SSL certificates that will serve a common domain name like app1.yourdomain.com and app2.yourdomain.com. Just ask your certificate authority about getting a certificate that will supply this need.
Finally, the IIS 7 and IIS 7.5 Internet Information Services Manager interface is picky about allowing you to manage host headers. In most cases, you will need to use the command-line tools described in the articles below.