This is dependant on how the web site is configured.
In web servers such as IIS and Apache, Web sites are accessed in one of 2 ways.
They either are accessed via IP; other wise known as having your site "bound" to an IP.
Or by a Host Header. A host header allows 1 IP to server multiple sites.
An IP can "only" do one or the other, not both.
So what does this mean?
If your site is using a shared IP. That is more than 1 website is hosted on that IP. Then simply add another host header to the site. The additional host header would be just abc.com (no host name in front of it)
If your site is "bound" to a single IP, then this is fixed in DNS. Simply create a A record that points to the root domain. The site will answer any name it receives because all it cares about is the IP.
The reverse is true with Host Headers. All the web server cares about is what host header it receives.