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ISPs are organised in a hierarchy where the Tier 3 ISPs are connected to Tier 2, and Tier 2 ISPs are connected to Tier 1 ISPs. Why are they organised in a hierarchy? Is it because if all devices were connected directly to each other, then the structure doesn't scale and becomes inefficient?

PS - I wasn't sure on which stack website to post this question, and this seemed like the best fit. However, if there is a website where this would be a better fit, please comment.

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    That is not necessarily true. Any ISP can contract to connect to any other ISP, just as any company can contract to connect to any ISP or any other company. ISPs are just companies like any other company. The tiers are sort of arbitrarily assigned. – Ron Maupin Mar 25 at 18:45
  • That makes sense, since I've AT&T in Tier 1 and Tier 3. So then, is there an advantage to arrange them in a hierarchical structure? – Ski Mask Mar 25 at 18:58
  • all devices were connected directly to each other is virtually impossible. You can't connect 10,000+ ISPs directly or billions of devices even. – Zac67 Mar 25 at 19:06
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There's not really a hierarchy. It's really more of a web. There are national (or international) ISPs that connect to a large number of other ISPs. There are smaller ISPs that connect to only a few other ISPs. Then there are large companies that have their own backbone (Google, Microsoft, Amazon, Facebook, etc).

There's no "rule" about who can connect to whom. Small ISPs do not have a national or international presence, so at some point they need to connect to larger ones to have world-wide connectivity. But they also can connect to other small ISPs. ISPs negotiate connectivity with each other as their business needs require. Since it costs money to "peer" with another ISP, each one tries to get the most connectivity for the least cost. The "Tier" designation is kind of arbitrary.

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  • Ah that makes sense. I thought that there was a literal hierarchy that was fixed. This clears it up. Thank you for your answer! – Ski Mask Mar 26 at 19:12
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(This will tread on "historical trivia")

The tiered construct was the original vision of the ARPANet (what became the internet.) The modern internet does not work like that. In fact, the early internet -- and ARPANet before it -- didn't entirely work like that either. With the advent of BGP, any network could be anywhere, connected to anyone. Instead of a tree, you have a web; it's not fully meshed, but it doesn't need to be.

Today, the "tier" designation is more a marketing tool than a hard rule. In general, a Tier 1 ISP has no transit peers; they run a "default-less" network. But even that is not always the case.

ISPs tend to build links where they need to be. If there's a lot of traffic to/from a specific network, it might be advantageous to directly exchange that traffic.

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