Using IIS, you can host multiple Web sites on a single server. There are a number of ways to distiguish between them but host headers allow you to host them on a single IP Address and Port.
When hosting multiple websites on a single IIS server, you can configure each Web site to use a different static IP address. This can be expensive as public static IP's are not unlimited in quatity. You can host them all on one IP/Port and host them over different ports but that is not very user friendly.
IIS allows you to assign multiple Web sites the same IP Address/Port and distinguish them from each other with host headers. When an IIS server receives a request for a Web page, it looks for the HTTP header which contains the actual domain name requested. IIS can then use this information to "route" the request to the proper Web site.
To create a host header for your Web sites, open up the Internet Services Management (IIS) MMC and right-click the Web site you wish to create a host header for and select Properties. Press the Advanced button next to the IP address (or All Unassigned).
On the Advanced Web Site Identification page, select the IP and click the Edit button.
Next to Host Header Value, enter the name by which the Web site will be located by.
Click OK and you will see the host header listed next to the IP. You can add multiple host headers to one site so that the same site is accessible with different names.
Repeat these steps for each of the Web sites. Once complete you can further secure the websites by redirecting IP requests, i.e. http://111.222.111.222 to an empty Web site.