One of the less well-known of the 802.11 family of standards is 802.11r. The point of this standard is to make switching between wireless access points fast. The range of an 802.11 access point is typically only a few rooms if inside, or about a hundred meters when outside. A person walking around may want to jump between a half-dozen access points in a few minutes. The 802.11r standard provides a mechanism for doing just that. If you've ever sat roughly an equal distance between two access points, you may have noticed that your connections periodically pause briefly as your networking stack decides to switch between the access points. The 802.11r standard improves this pause time, dropping the switching time down to well under a second. This change makes 802.11 feasible for mobile telephone use. In an area with a load of .11r-supporting access points, you can walk around while talking on the telephone, and not notice when your call jumps between those points—just as you don't notice now when your mobile phone jumps between towers. (Well, in theory you don't notice; in practice, you probably do).
This kind of handoff works well if all of the access points are on the same network segment. However, imagine walking down a street full of open access points configured in a mesh (802.11s). When you start at one end, you make a phone call and it's routed through the first house's network connection. As you walk down the street, your call is seamlessly handed off between access points until you get to the far end of your travels. At this point, your call's packets are being relayed via a dozen access points to the far end, which isn't very efficient. Ideally, before this point you would use the new access point's uplink. Unfortunately, each access point has a different routable subnet address, so if you switch to the new access point's uplink you'll suddenly switch IP addresses, which will break your connection.
This is where Mobile IPv6 comes in. IPv4 also had a mobile variant, which relied on triangular routing—in simple terms, the old network acted as a relay for packets, which added latency every time you moved. With IPv6, IPSec allows the routing tables to be updated securely. Currently, the switching time is around a second, which isn't quite fast enough for VoIP traffic, but this time is expected to improve.
With Mobile IPv6, the boundaries between networks become a lot more fluid. If you're sitting in a coffee shop with free WiFi, you may want to use the coffee shop's network, rather than paying for use of your cellular provider's network. If you walk in while making a call, you would want to switch automatically to using the coffee shop's free WiFi rather than the cellular network, which is a more scarce resource and likely to be more expensive.