Not every time. Sometimes it's DNS.
That's a lesson many don't know.
Was fixed in incredible coincidence manner, too - the CTO of the network link provider was in their offices (in the same building as me) and felt bored. Apparently having went through all the levels from hauling cables in datacenter up to CTO level, after short look at traceroutes he just picked a phone, called NOC, and ordered a line card replacement on the router :D
Some router near a construction site had dust settle into the gap between the laser and the fiber, and it attenuated the signal enough to see 40-50% packet loss.
We figured out where the loss was and had our NOC email the relevant transit provider. A day later we got an email back from the tech they dispatched with the story.
Really complex systems (the Web) also fail because of caching.
It's funny that some folks claim DNS outage is a legitimate issue in systems whose both ends they control. I get it; reimplementing functionality is rarely a good sign, but since you already know your own addresses in the first place, you should also have an internal mechanism for sharing them.