Hello! A CDN is just a webserver, with some extra configuration at the network level.
If you're unfamiliar with how BGP routing works between Internet operators, all you have to know is that each operator (AS - Autonomous System) has links to others, and can decide:
- where through to route its own traffic to other AS (eg. your AT&T ISP may have a direct route to OVH (where lemmy.ml is hosted), or may go through a 3rd party transit provider like Level3 (for a much higher cost)
- in which Points of Presence (PoPs) to advertise which routes to its own network ; you can advertise address 1.2.3.4 only from your Paris PoP and let other AS find the best route to you, and at the same time advertise address 1.3.1.2 from Paris, Moscow, Beijing, etc..
A CDN is more or less an IP address announced from many places, pointing to different servers depending on where you access it from, but with a single TLS certificate for HTTPS. So if you try to reach it from AT&T, you'll get traffic directed to a datacenter in USA, etc..
So, on a system level, it's REALLY easy to deploy a CDN. It's just a webserver. On a network level, it's more complicated because you need your own IP block that's announced on your own network. In fact, that's not entirely true as you can find (very costly) network operators that'll do that for you.
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