Configure alternate access mapping

Each Web application can be associated with a collection of mappings between internal and public URLs. Both internal and public URLs consist of the protocol and domain portion of the full URL (for example, https://www.fabrikam.com).

A public URL is what users type to get to the SharePoint site, and that URL is what appears in the links on the pages. Internal URLs are in the URL requests that are sent to the SharePoint site. Many internal URLs can be associated with a single public URL in multi-server farms (for example, when a load balancer routes requests to specific IP addresses to various servers in the load-balancing cluster).

Each Web application supports five collections of mappings per URL; the five collections correspond to five zones (default, intranet, extranet, Internet, and custom).

When the Web application receives a request for an internal URL in a particular zone, links on the pages returned to the user have the public URL for that zone. For more information, see Plan alternate access mappings (Office SharePoint Server).

Manage alternate access mappings
On the top navigation bar, click Operations.
On the Operations page, in the Global Configuration section, click Alternate access mappings.
For information about how to perform this procedure using the Stsadm command-line tool, see Addalternatedomain: Stsadm operation (Office SharePoint Server).

Add an internal URL
On the Alternate Access Mappings page, click Add Internal URLs.
If the mapping collection that you want to modify is not specified, then choose one. In the Alternate Access Mapping Collection section, click Change alternate access mapping collection on the Alternate Access Mapping Collection menu.
On the Select an Alternate Access Mapping Collection page, click a mapping collection.
In the Add internal URL section, in the URL protocol, host and port box, type the new internal URL (for example, https://www.fabrikam.com).
In the Zone list, click the zone for the internal URL.
Click Save.

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To Get that please follow the steps as shown below
1) Ask the Systems team to place a DNS entry called portal
2) On the SharePoint installed server go to C:\WINDOWS\system32\drivers\etc\Hosts . Open the hosts file with notepad and enter the IP address of the SharePoint server, give space (or hit the tab button once) and then type the DNS name which is portal (e.g 172.18.00.00 portal)2) On the same server open IIS (start-->RUN-->Inetmgr-->Enter)
3) Expand the folder web Sites and right click on the site and select properties. On the tab "web Site", below web site Identification click on the button Advanced.
4) Under Multiple Identities for the web site click on ADD, then in the TCP port enter 80 and in the Host Header Value type port, then click ok, ok and apply.
5) go to the central administration and click on Operation, under Global configuration select "Alternate Access Mapping" in here click on the web application (in our case http://servername:7788) you will be taken to a next page called Edit Internal URLs in here just click the cancel button, when you do that you will be taken to another page called Alternate Access Mappings, in here please click on Add Internal Urls, you will now be taken to another page called ADD Internal URLS . make sure that you have selected the right site collection on the right side, then in the Add Internal Url type http://portal and click Save.You can now open a browser and type portal and hit enter. you will be taken to the site http://portal/pages/default.aspx.