a tier-1 ISP

The Internet has a loose hierarchy of networks. At the top sit a handful of giant backbone networks called tier-1 ISPs. The defining property is simple but strict: a tier-1 ISP can reach every other network on the Internet purely through settlement-free peering, without ever paying anyone for transit. Everyone else, lower down, has to buy transit from someone above them. Picture the global airlines that fly everywhere on their own routes and only ever swap passengers with each other, never buying a seat from a smaller carrier.

Below the tier-1s are regional and national providers (often loosely called tier-2), who peer where they can but still BUY transit from one or more tier-1s to reach the parts of the Internet they can't peer to directly. Below them are access ISPs (sometimes tier-3) — the networks that actually connect homes and businesses, and that buy transit from the layers above. Content providers and big platforms (Google, Meta, Netflix, Amazon) sit somewhat outside this ladder: they run vast networks of their own and peer aggressively, getting their content close to users rather than selling transit. So the picture is less a clean pyramid and more a densely interconnected mesh with a transit-free core.

The tier-1s together form that transit-free core: because each can reach everyone through peering, and they all peer with one another, every destination on the Internet is reachable through the union of their routing tables. That is also why they sit in the default-free zone — they carry full routes to everywhere and never need a default route pointing "upward," because there is no one above them.