It appears that 2600:: no longer responds to ICMP. $ mtr -rwc 1 2600:: Start: 2024-04-06T10:53:41+0100 HOST: metropolis Loss% 1.|-- lcy02.flat.b621.net 0.0% [...] 6.|-- ldn-b4-link.ip.twelve99.net 0.0% 7.|-- ldn-bb1-v6.ip.twelve99.net 0.0% 8.|-- nyk-bb2-v6.ip.twelve99.net 0.0% 9.|-- ??? 100.0 10.|-- sprint-ic301620-nyk-b5.ip.twelve99-cust.net 0.0% 11.|-- ??? 100.0 This seems to have happened around Friday 5th 13:40 UTC. 2600::, a IP address owned by the Sprint network (Now since acquired by Cogent Communications) is a common (at least in my circles) IPv6 testing address, in a similar way that 8.8.8.8 or 1.1.1.1 is for a quick address to remember that always pings, when such a address is so easy to remember, you sometimes cannot help it becoming a "core project" :) ( https://xkcd.com/1361/ ) 2600:: is also used to be the address of sprint.net, now sprint.net has no v6. This is sad, and I would either propose that Cogent/Sprint (I assume 2600:: is under the ownership of Cogent now) revive this address as it's a very helpful testing address that is burned into the minds of many. Or at the very least, I'm more than willing to tank the effort of responding to ICMP!