The Internet is not a single, centrally controlled network but rather a massively distributed, decentralized, and hierarchical collection of interconnected, independently managed networks. This organizational structure is fundamental to its immense scale, resilience, and the diversity of services it supports.

The core organizational unit of the Internet's routing architecture is the Autonomous System (AS). An AS is a collection of IP networks (and the routers that connect them) that are under a single, unified technical administration and control, and that present a coherent, common routing policy to the outside world. Each AS is assigned a globally unique Autonomous System Number (ASN) by regional Internet registries (e.g., ARIN, RIPE NCC, APNIC). Examples of ASes include: a major university campus network, a large multinational corporation's global private network, a government agency's network, or, most commonly, an Internet Service Provider (ISP).

These are the networks that ultimately consume Internet services. They encompass a vast range of entities, including: Residential Users: Typically connect to a local ISP (Tier-3 or Tier-2) via technologies like DSL, cable, fiber, or wireless. Their home network is usually managed by the ISP's provided router/modem. Small to Medium Businesses (SMBs): Connect to local or regional ISPs. They may have a small internal network and a router that performs basic routing and NAT. Large Enterprises/Universities/Governments: Often have very large, complex internal networks. They typically operate their own AS(s) and manage their internal routing infrastructure. They establish connections (peering or transit) with one or more ISPs to gain access to the rest of the global Internet.

ISPs are the backbone of the Internet. They are organizations that own and operate the vast network infrastructure (routers, switches, fiber optic cables, wireless towers) and provide Internet connectivity to end-user organizations and to other ISPs. ISPs are themselves organized in a hierarchical structure: Tier-1 ISPs (Global ISPs): These are the largest, highest-capacity ISPs with truly global network footprints. They interconnect with virtually every other Tier-1 ISP and form the core mesh of the Internet's backbone. The defining characteristic of a Tier-1 ISP is that it does not pay for transit to any other ISP; instead, it engages in settlement-free peering with all other Tier-1 ISPs. Examples include Lumen Technologies (formerly Level 3), NTT, AT&T, Verizon, Zayo. Tier-2 ISPs (Regional/National ISPs): These ISPs have regional or national network coverage. They connect to and exchange traffic with other Tier-2 ISPs (via peering) and, crucially, connect to Tier-1 ISPs (for which they typically pay for transit). They also provide connectivity to smaller Tier-3 ISPs and directly to large end-user organizations. They are the 'middlemen' of the Internet, buying transit from Tier-1s and selling it to smaller entities. Tier-3 ISPs (Local/Access ISPs): These are the smallest ISPs, providing direct Internet access to end users (homes, small businesses) within a limited geographic area. They connect to (and pay for transit from) Tier-2 or sometimes Tier-1 ISPs to access the rest of the Internet. They do not typically peer with other ISPs; their primary function is last-mile access.

Peering: A direct, often bilateral (two-party) agreement between two ASes to exchange traffic freely between their respective networks. Peering typically occurs at Internet Exchange Points (IXPs), which are physical locations with high-speed interconnections where multiple ASes can meet and peer, or via direct private interconnects. The primary motivation for peering is mutual benefit – keeping traffic local, reducing latency, and avoiding the costs associated with buying transit from a third-party ISP. Peering agreements are usually 'settlement-free' (no money changes hands for traffic volume). Transit: A commercial relationship where one AS (the customer or 'stub' AS) pays another, larger AS (the provider or 'transit' AS) to carry its traffic to any destination reachable via the provider's network or its peering partners. This is how most smaller ASes and end-user networks gain full Internet connectivity. The provider is essentially selling access to its global routing table.