20 2因特网 23因特网交换点( nternet eXchange Point,P) POP #3 Tier 1 Networks Tier 2 Networks IXP POP #2 Tier 2 ISP 菌 Tier 3 Network Tier 3 Network (multi-homed ISP) (single homed ISP) Internet users (business, consumers, etc) 河南中医药大学/阮晓龙713938213680/http://network.xg.hactcm.edu.cr