The Challenge description says, we need to give the IPv6 address of the DNS root server with the least Autonomous System Number.

To solve this challenge one must know that there are only 13 DNS root servers in the world.
Each server is not just one computer but a cluster of computers, and the number 13 was chosen as a comprimise between reliability and performance.

Read More here:

[https://www.lifewire.com/dns-root-name-servers-3971336](https://www.lifewire.com/dns-root-name-servers-3971336)

Their IPv4 and IPv6 addresses are publicly available on this site.

[https://www.iana.org/domains/root/servers](https://www.lifewire.com/dns-root-name-servers-3971336)

The AS Numbers of the IPv4 addresses, can be identified using many tools, I used,

[https://hackertarget.com/as-ip-lookup/](https://hackertarget.com/as-ip-lookup/)

The IPv6 address of the DNS server with the smallest AS Number, is the flag.