先了解一下基础的四层网络结构：
然后了解一下 Peering（视频里面提到了 Tier 1 network）
所以最核心的一些使用 Peering 政策的网络供应商，就可以称为 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.
在这个网站上可以查询到所有网络供应商的 Peer 政策：
