Comment Name-based virtual hosts and TLS/SSL (Score 1) 665
Most web sites run on name-based virtual hosts. This allows multiple web sites to use the same instance of the web server (Apache, IIS, etc.), reducing costs.
This presents a chicken-an-egg problem with TLS/SSL (the encryption used for https).
When the web server receives the initial request from the browser, it sends back a certificate for it's domain that says to the browser, "Yes, I am really where-ever.com, because I paid money to GoDaddy, Comodo, Verisign or whoever and they'll corroborate."
The problem is, when that first request comes in, and you are using TLS/SSL and name-based virtual hosting, the server can't read what domain name was requested to present the correct certificate. You haven't finished negotiating the TLS/SSL connection yet, so you can't read the URI embedded in the request.
So, you need a different IP for each domain that you are going to serve (IP addresses are becoming rare) or use some other hack to accomplish this.