SharePoint 2010 : Alternate Access Mappings Programmatically

here is the good post on this

http://www.directsharepoint.com/2012/01/get-current-web-application-alternate.html

or

            foreach (SPAlternateUrl altUrl in oWebApp.AlternateUrls)

            {

                if (altUrl.UrlZone == SPUrlZone.Default)

                {

                    incomingURL = SPUtility.AlternateServerUrlFromHttpRequestUrl(altUrl.Uri).AbsoluteUri;

                }

            }