FreeBSD has the distinction of being the first OS to support IPv6 (in the Kame project under the parent WIDE project). The TCP/IP network stack implementation in FreeBSD is one of the best anywhere, and is still the reference implementation for anyone interested in implementing IPv6 on other operating systems. The same high quality IPv6 stack is deployed on NetBSD and OpenBSD.
FreeBSD is a direct lineal descendant of the Berkeley System Distributionof AT&T UNIX, v4.2. AT&T was prohibited from selling operating systems (as a public Utility), but they released the entire source code to several universities. The University of California at Berkeley added a lot of new functionality into the AT&T version, including virtual memory and TCP/IP. They rewrote so much of it that they wanted to release it to the public for free. AT&T tried to prevent Berkeley from releasing their branch of UNIX, but the court ruled that most of the BSD version had been written or heavily modified by the University of California at Berkeley group, so if they would rewrite the remaining small amount of original AT&T code, they could do anything they wanted to with the resulting OS (except call it UNIX), despite the fact that it started life as AT&T’s intellectual property. The BSD license allows anyone to do anything they want with the code (so long as attribution to UC Berkeley is made). FreeBSD is at the heart of many great open source and commercial products.
Here is a recipe to help you deploy the current most stable version of FreeBSD with full dual stack (IPv4 + IPv6) support in VirtualBox (or hardware). You can also deploy it with only IPv6 if you wish.