Definition
IPv6, finalized in 1998 and slowly deployed since, is the next-generation Internet Protocol. Its addresses are 128 bits, providing approximately 340 undecillion (3.4 × 10^38) possible values — effectively unlimited for any foreseeable use. IPv6 also includes mandatory IPSec support, simplified packet headers for faster routing, and built-in autoconfiguration that eliminates much of the manual setup required by IPv4.
Adoption
After two decades, IPv6 deployment is significant but not yet universal. Major mobile carriers, large content providers (Google, Facebook, Netflix), and modern home ISPs in many countries deliver IPv6 routinely; older home networks and many enterprise environments are still IPv4-only. The result is a long transitional period where most networks support both protocols simultaneously — 'dual stack' — which creates the conditions for IPv6 leaks.
VPN Implications
If your device has IPv6 connectivity but your VPN tunnel only supports IPv4, traffic to IPv6-reachable destinations may travel outside the tunnel, exposing your real IPv6 address. Modern VPN clients address this by either tunnelling IPv6 explicitly, blocking it while the VPN is connected, or providing IPv6 servers as part of the VPN service. A VPN that does not address IPv6 in some way is increasingly considered a leak risk.