tier 1

A Tier 1 network is an Internet Protocol (IP) network that can reach every other network on the Internet solely via settlement-free interconnection (also known as settlement-free peering). Tier 1 networks can exchange traffic with other Tier 1 networks without having to pay any fees for the exchange of traffic in either direction, while some Tier 2 networks and all Tier 3 networks must pay to transmit traffic on other networks.
There is no authority that defines tiers of networks participating in the Internet. The most common and well-accepted definition of a Tier 1 network is a network that can reach every other network on the Internet without purchasing IP transit or paying for peering. By this definition, a Tier 1 network must be a transit-free network (purchases no transit) that peers for free with every other Tier 1 network and can reach all major networks on the Internet. Not all transit-free networks are Tier 1 networks, as it is possible to become transit-free by paying for peering, and it is also possible to be transit-free without being able to reach all major networks on the Internet.
The most widely quoted source for identifying Tier 1 networks is published by Renesys Corporation, but the base information to prove the claim is publicly accessible from many locations, such as the RIPE RIS database, the Oregon Route Views servers, Packet Clearing House, and others.
It can be difficult to determine whether a network is paying for peering or transit, as these business agreements are rarely public information, or are covered under a non-disclosure agreement. The Internet peering community is roughly the set of peering coordinators present at the Internet exchange points on more than one continent. The subset representing Tier 1 networks is collectively understood in a loose sense, but not published as such.
Common definitions of Tier 2 and Tier 3 networks:

Tier 2 network: A network that peers for free with some networks, but still purchases IP transit or pays for peering to reach at least some portion of the Internet.
Tier 3 network: A network that solely purchases transit/peering from other networks to participate in the Internet.

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