I've done some more research on this.
This morning two main net cable backbone providers had serious connectivity problems.
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9 hours ago.
and this has been also the cause for some people having trouble accessing POTN recently. When specific backbones get bad, DNS routing should change to avoid those backbones (e.g. http://www.nthelp.com/maps.htm
) and I have had reports that routers have not been changed when needed. Routing is changed mainly by DNS server provider and backbone provider. They know about these issues and work to solve them.
That is why some people have trouble accessing POTN, or get very slow speeds - it all depends how DNS servers route the path from their IP to POTN IP.
For example the problems few days ago were due to AT&T broken cable. The routing should have reflected this ASAP but it did not.