When it comes to IP, the Internet Protocol, there are two versions to choose from. One is IPv4, which is currently the most common. The other is IPv6, which is newer, and is growing in popularity.
One of the methods of migrating to IPv6 is called dual-stack. This means that a device is using two different versions of the TCP/IP stack, side-by-side. One version is using IPv4, and the other is using IPv6.
Running both versions at once makes it easier to implement IPv6. This way, all devices can begin to support IPv6 over time, and IPv4 can be turned off later.