With all the IPv4 addresses running out, with the multitudes of devices out there that each have their own separate IP addresses, IPv6 handles that nicely. IPv4 can hold ~4.2 billion addresses, but IPv6 can handle ~340 undecillion addresses. However, with anything new in techonology, theres always the chance for security issues.
IPv6 was set up to try to fix many of the issues that its predecessor had, but one issue with IPv6 will be how all routable devices will obtain an address.
Also, with many of our devices able to connect to the internet now, we may all be tracker using our IP addresses. A simple ping or trace route can send packets of information back to whomever wants to know where exactly we are. Many smart phones have the same issue (like Apples iPhone) with their GPS implementation. Privacy will be a major concern, as companies aim to track end users data for research, or even someone just maliciously trying to keeps tabs on someone. Anonymity will be faced with some major challenges.
By default, IPv6 actually releases a products MAC address. A MAC address is a static number, much like a serial number, that is unique to each individual product. By acquiring a MAC address, you can find out who exactly owns the product. And by pinging or using trace route, you can know exactly where “John Doe” is at any time.
This issue has been recognized, as a research team devised Moving Target IPv6 Defense (MT6D), which is a way for people to communicate over the internet while still maintaining there anonymity to an extent. However, nothing is 100% in security, so it will be interesting to see how the “bad guys” try to find a way around this.