So this is probably a bit off topic because it's more network-related, but I'd appreciate some pointers on how to troubleshoot:

- When I try to open https://mirrors.slackware.com from a browser or connect to it with Slackpkg, it times out.
- I can ping it, traceroute it and get the correct IP address from a local nslookup (207.223.116.213).
- Entering the IP address in a browser also times out.
- No other website I use is affected in this way (not even www.slackware.com or https://docs.slackware.com).
- I have no issues connecting to it from outside my network (via mobile).

I'm on a ds-lite connection if that's relevant, but can connect to other ipv4 and ipv6 addresses with no issue.

Where can I look next to fix this? Any help is appreciated!

I can access mirrors.slackware.com without issue, so I would say it's a local issue to your ISP.

Try my (very fast) mirror: https://slackware.uk/

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No problem here. Put Wireshark on the interface and see what happens when you try to connect.

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Thank you, I hadn't even considered this.

So, The difference between this site and others seems to be that it has no AAAA record, and no CNAME record that points to a host with an ipv6 address.
So mirrors.slackware.com can't be accessed by ipv6 and I can only connect to hosts with an associated ipv6 addresses from my network. Looks like I'll have to call my ISP.