Information is routed through the Internet in small packets, and a traceroute tool can check the path that one packet followed.
To comprehend how routing works and what the traceroute tool does, readers need to understand that all information sent or received on the Internet is just a small piece of the original data. For example, when requestors visit a Website and they want to retrieve a Web page, the server of that Website receives the request for the Web page and sends the Web page to the requestor. The requestor does not receive the whole Web page all at one time; instead, it is divided into little pieces of information called packets. These packets reach the requestor by traveling through the Internet and passing through computers along the way.
Each packet is like a letter, in that it has a sender and a receiver. Computers connected to the Internet use a packet-switching technique to transfer packets from one system to another. The packet is, essentially, handled as a “hot potato”; that is, the sending computer (for example, the server of the Website the requestor is visiting) sends it to the closest router. This router receives the packet and looks at the recipient address. If the recipient address belongs to a computer in the same network segment as the router, the router delivers the packet to this computer and the process stops. If the recipient address is not correct, the packet is sent on to the next nearest router. If the recipient address is still not correct, the packet is sent on to the next nearest computer. The cycle continues until the packet reaches the receiver with the correct recipient address. The Web page may pass through routers in several countries before it reaches the right requestor with the right address. Routing tables stored in each router assist in the process of determining the “next nearest” router.
Also, if some routers along the way are down, the data will take another active path. Some routers may be found to be too busy or too crowded, so they will take quite some time to respond. For this reason, the traceroute tool was developed. This tool, which can check the path that one packet followed, can be used by system administrators not only to discover the path taken but also ascertain the amount of time the packet took to reach the correct address recipient.
Every IP packet has a field named TTL (TimeToLive), which can take values between 0 and 255. Each router processing the packet looks at this value and subtracts 1 from it. This procedure continues until the content of the TTL field is decremented to contain 0 or 1. When the TTL field has reached 0, the router drops the packet. Such a mechanism is needed to keep a packet from traveling on forever, never finding the correct receiver.
Silvestri, M. Traceroute Tools. [Online, 2000.] Wowarea Website. http:// www.wowarea.com/english/researches/wg4_traceroute.htm.