A firewall separates the company internal network (intranet) from the Internet. This enables intranet clients to gain information from the Internet, while preventing any aggressors on the Internet from attacking the computers of the internal network.

Let us say that a company has been assigned the company.com domain. It will want to use this domain for both the Internet and its intranet. The company.com domain in the Internet will most likely contain only a few records such as www.company.com, mail.company.com and a few other records (MX records for company.com pointing at mail.company.com, etc.). The company.com domain on the intranet can contain, on the other hand, tens, hundreds, or even thousands of computers.

To put this differently, there will be two company.com domains, with each of them containing different records, but the problem is that they both will have the same company.com name. There cannot be two domains of the same name on the Internet. But both...