system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the advert play perfectly, but the video takes an eternity to buffer?

I understand the ad is usually considerably shorter than the video, but the buffer times are often not relative. It feels like they have two totally separate sources. I notice this less on YouTube, but more so on VoD services (namely 4oD and BBCiPlayer)  

Edit: two  


Edit 2: So I got a PM telling me a comment from this thread had been Best Of'd. So I think to myself someone must have the perfect answer that people are finding really useful.  

Nope  

Just Aunt Marjorie's Honey Baked Possum recipe :)
gpt
Ads are generally served from a separate server or Content Delivery Network (CDN), as you guessed. Content providers contract with ad providers like Doubleclick or Freewheel to fill ad slots. When you play a video, the player makes a call to the ad provider, which serves an ad from their own network.