Trilight Zone

[digital8]

tracert stands for traceroute. This Unix command will show you what route your packets will have to go through until they will reach the given IP address / hostname.
How this works: every packet has a value in it that is called TTL. TTL stands for Time To Live. This value is decreased every time a packet goes through a router. When the TTL hits zero, the packet is discarded and an ICMP error is sent back to the sender of the packet, telling him what happened and that he should resend the packet with a higher TTL (the recommended TTL these days is 64). This is done in order to prevent packets from getting lost and looping across the Internet.
Now, what tracert does is quite simple - first, it sends a packet to the given IP or hostname with TTL=1. The packet takes one step and then dies. The first router informs us what happened. Since the error packet, like every other packet, contains the IP of the sender (the first router, in this case), we know who he is. Next, a packet with TTL=2 is sent. The second router returns it and tells us what happened, so we know who the second router is.
Every time the TTL is increased by one, until the packet finally reaches it's destination. Meanwhile, tracert builds a list of routers that passed this packet along.
Tracert is quite useful, since it can tell you a lot about a given IP or a given hostname. For example, if you entered an IP that does not have a hostname, you could traceroute it instead and see who's the ISP of that IP (the last routers in the list should belong to this IP's ISP). You could also use it to find out who's the ISP of large websites in the same way.
For more information, type 'man tracert' (without the quotes) on a Unix system.