I am a bit baffled by the arguments about what the author meant. (Needless to say, people who “prove” they can run `traceroute yahoo.com` are hopeless.) Tracer

I am a bit baffled by the arguments about what the author meant. (Needless to say, people who “prove” they can run `traceroute yahoo.com` are hopeless.) Traceroute is a byproduct of a complex process that may match the forward path of real application traffic you send to some address. It may not if you have some local (transparent) proxy for certain protocols, or QoS in the middle (that may mis-classify your application traffic, unlike regular ICMP pings), or intrusion detection system, DoS protection system, or low level load balancer on the other end. The author invites to check another explanation multiple times, and it gives at least one example when replies from a certain hop are canonically sent by a different network hop. Some ISPs use IP addresses from private ranges, and they surface in traceroutes. Some ISPs lease others' infrastructure, and you see those IP addresses and DNS names inside their networks. Some ISPs deliberately mess with low TTL packets to cloak details like those from clients and competitors, and keep their physical network layout secret. You need to know all of that, from ARP to BGP, to understand traceroute output. You also need to remember that one post in network engineering community a month ago that told that ISP M was bought by Corp N to guess that they are probably switching traffic from leased line between IX A and IX B to Corp N's own lines between City X and City Y, which might explain what you see. However, traceroute manual doesn't give you a list of 30 thick books to read in advance, nor does it contain the latest gossip about the state of ISP O's hardware in City Z. Therefore, unprepared user is not aware of all the possible complexities. This reminds me of “helpful” websites that state that hosts with IP addresses of well known CDNs are located in California. Then people from all other the world run ping or traceroute, and deduce that they have a 10 ms RTT to California. Wow, such a great CDN!