Lists like pto.txt (often shorthand for "protocol" or "point-to-point") are frequently used by security researchers to document reachable IPv6 nodes for capability testing. Highly Recommended IPv6 Blog Resources
If you are working with that list of IPs, keep these standard prefixes in mind: : Reserved for documentation and examples. 14K IP.v6 pto.txt
: Essential for understanding how IPv6 scan traffic works and how "hitlists" of addresses are generated. Lists like pto