Currently sharepoint responds to http://sharepoint.mydomain.com. I want it to respond to http://portal.mydomain.com also. This is an intranet site.
Is a DNS alias and alternate access mapping all I need to do? If not what are the necessary steps to accomplish this?
It seems so. Take a look at this: http://social.technet.microsoft.com/Forums/en/sharepointadmin/thread/b76cf17d-85cb-4a5e-9f7b-e0efdf438667 and this: http://dotnetengineer.wordpress.com/2008/06/30/url-aliases-for-your-sharepoint-site/. According to the second link you may want to set Zone setting to Extranet.