You can create host named site collections if you would like to host all of your portal sites as site collections within one SharePoint web application.
· provide a fully qualified domain name (FQDN) for your SharePoint web application through the alternate access mapping in SharePoint Central Admin (Make sure DNS entry or host file entry made to resolve, leave host header blank at IIS site level)
· Create Site Collection using “stsadm” utility instead of central admin application and use the “–hostheaderwebapplicationurl” switch. This will create the site collection as host named site collection
Advantage of doing this would be you don’t have to manage too many content databases, IIS web sites etc. Probably not a good approach if you want to isolate content within site collections in separate content databases.