If we have a single physical box (single IP address), how could it support multiple domain names, for example, I want to support domains in two patterns,
For pattern one you only need to register "b.com", this is your host domain.
a1.b.com, a2.b.com and a3.b.com are sub domains of b.com, you can create these from your host servers control panel. Doing so should create a new directory folder for each sub domain along side your world wide web (or www) folder. Each sub domain can have it's own content
for pattern 2 though, multiple domains can be registered to the same public IP address but across the board all the content would be the same as they would be accessing the same directories.
Initially, all these domains and subdomains would need to pointed at the IP address through DNS.
For web pages, almost any web server can do this, the exact implementation/configuration depends on the web server you choose to run. For example, for Apache, you can find some excellent documentation on their website at http://httpd.apache.org/docs/2.2/vhosts/examples.html
Ultimately, they all work the same way, a client makes a connection to a server and requests "a.com" so the web server will pull files from directory "a". If they request "b.com" the web server will pull files from directory "b".
