cket handling and to limit the bandwidth cost of the IPv6 header. Improved support for extensions and options Changes in the way IP header options are encoded allow for more efficient for- warding, less stringent limits on the length of options, and greater flexibility for introducing new options in the future. Flow labeling capability A new capability has been added to enable the labeling of packets belonging to particular traffic flows for which the sender requests special handling, such as nondefault quality of service or real-time service. Authentication and privacy capabilities Extensions to support authentication, data integrity, and (optional) data confi- dentiality have been specified for IPv6. IPv6 includes some significant changes to the headers at the IP layer. I suggest reading the RFCs to see what those changes are. Instead of boring you with packet diagrams, I'm going to focus on what you should know to get started with IPv6 and how you configure IPv6 on a router. Addressing IPv6 addresses are a bit more complicated than IPv4 addresses. To start with, where IPv4 uses only 32-bit addresses, IPv6 uses 128-bit addresses. That equates to 3.40292367 X 1038 addresses. That's roughly 295 IP addresses for every person on Earth in 2011. In other words, that's a lot. For you number geeks out there, 3.40 X 1038 is 340 undecillion (http://en.wikipedia.org/wiki/Undecillion). Really, though, the address scheme was designed to support efficient route aggregation. Subnets in IPv6 are supposed to contain 264 addresses. To put that number in per- spective, consider that all the IP addresses in the IPv4 universe would consume 1/64th of the address space available in just one IPv6 subnet. Like I said, there are a lot of addresses in the IPv6 space. IPv6 addresses don't use decimals like IPv4, and that is the first hurdle many people struggle with using IPv6. IPv6 addresses are composed of groups of four-digit Addressing hexadecimal numbers separated by colons. Luckily, leading zeros can be eliminated within each set of