Для тебя вот цитатка:
Tier 1 Service Provider : A network, which does not purchase transit service from any other network, and therefore peers with every other Tier 1 network to maintain global reachability. They are the biggest guys geographically, but not always the number of customers point of view.
Tier 2 Service Provider : A network with transit connections , customers and some
peering, but that still buys transit service from Tier 1 Providers to reach some portion of the Internet.
Tier 3 Service Provider : A stub network, typically without any transit customers, and without any peering relationships. They generally purchase transit Internet connection from Tier 2 Service Providers, sometimes even from the Tier 1 Providers as well (I know some non-profit organisations which have a transit connection from Tier 1)