I listened recently to the Security Now! podcast on “How the Internet Works Part 1″ (episode 25) and learned about a traceroute.

I could have just read the WikiPedia entry for traceroute I suppose, but I had just heard the term and not thought much of it. I knew a traceroute is an internet command you can send from your computer that will tell you how many “hops” on different Internet routers the packets are taking between you and a defined endpoint.

The way this works is ingenious. Every packet on the Internet has a “time to live” value, and each time the packet encounters a router it subtracts ONE from the current “time to live” value. When the value reaches zero, it sends back an “ICMP” packet to the sending IP address letting it know it did not reach its destination, and providing the IP address of that last router. A traceroute works by first sending a packet to a desired destination IP with a “time to live” of just one, so it is immediately sent back as an ICMP packet by the first router. Then it sends another packet, but increments the “time to live” value up by one. In this way it eventually discovers the IP addresses of all the routers which are on the path (navigated at that moment) between the computer originating the traceroute and the IP address of the computer that is the defined destination.

This security now podcast provides some great background information about the overall architecture of the Internet, and I would highly commend it to anyone interested in learning more about how the Internet works and how to troubleshoot issues related to it.

To run a traceroute in Mac OS X (without using the terminal), launch the Network Utility program found in Applications – Utilities. Click the traceroute tab at the top, and enter the IP address or domain name of a website or computer you want to run a traceroute to. Then click TRACE. I did a traceroute from my current location to my Speed of Creativity website hosted by POWWEB, and it took 15 hops one way to get there.


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