It's been a strong year for shareholders in VeriSign ( VRSN) -- Berkshire Hathaway included. Berkshire added 3.69 million shares of VRSN to its portfolio as a new position last quarter, and it more than doubled its stake in the most recent quarter, piling on another 4.49 million shares. That gives Buffett and company a 5.35% ownership stake in VeriSign's outstanding shares. In a nutshell, VeriSign's business is the Internet. VeriSign is one of the biggest Internet infrastructure firms in the world, maintaining a central directory of all domains and many of the systems that run them. The company is essentially the gatekeeper to owning a .com Web address, and as a result, VeriSign is able to collect a fee for each of the more than 110 million domains that fall under its purview. While the introduction of a new category of "top level domains" (such as .aero for aviation businesses or .mobi for mobile websites) does increase the competition faced by VRSN, the new naming options aren't likely to steal share from the dot-com domains. Instead, firms are likely to just register both names. VeriSign's exclusive contracts with ICANN offer an attractive economic moat for the firm. Because increases in registration fees are built into those contracts, VeriSign will be able to generate attractive growth over the course of the next several years without needing to work for it. To see the rest of Berkshire Hathaway's plays -- including a complete list of which stocks the firm added or sold off, check out the Warren Buffett Portfolio on Stockpickr. -- Written by Jonas Elmerraji in Baltimore.