Now when we run this command the first thing thats

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Now when we run this command the first thing that's going to happen is that traceroute is going to try to resolve into the appropriate IP address and then it will send ICMP echo request packet, just like ping did, to whatever IP address this host resolves into. But the TTL parameter is going to be set to a value of zero. That way the first router that receives that ping packetis not going to forward it on to the destination host, instead it's going to say, "Hey, the TTL on this has expired, I'm dropping it." Error message back to the sending host. Traceroute's going to get that error message, which will have the IP address of the router that dropped it contained within it. And by doing this, traceroute can see the IP address of the first hop router between this system and this system. Then the next packet that it's going to send is going to have a TTL of one. That way it will pass that first router, which will then decrement the TTL from one to zero and when that packet arrives at the next router then that router is going to send an error message back to the system here that's running traceroute saying, "Hey, I had to drop this packet, here's my IP address." And by doing that we now know the IP address of the second router between this system and this system and it'll keep doing that incrementing the TTL parameter by one for each ICMP echo request packet being sent to thishost until it finally reaches this destination host. By doing this we essentially create a map of routersbetween this host and this host. Works really nice. Let's go ahead and give it a try. We'll run traceroute . You can see that the first router is gateway.nebo-tech.com, it has an IP address of 10.0.0.1. Now the second host you can see here, we got a bunch of stars with request timed out. That doesn't mean that the router is necessarily bad it just means that the traceroute command couldn't actually resolve its IP address into a host name and therefore it just put a request timed out error message right here. Now we can see the rest of the routers between this system here where traceroute is being run and . And the number of the hop is shown over here, hop one, hop two, three, four, five, six, seven, down to 17. So we know that there are 17 routers between my system here running traceroute and the web server on the internet with a host name of . And notice over here we also have round trip times for each of these hops, you'll notice that there are three. What actually happens when you run the traceroute command is that it actually sends three packets with the same TTL number, that way we can measure the average round trip time for one particular host. As you can see hop one was less than one millisecond because this is the local gateway router. However, as we went out on the internet the round trip times started taking longer. For example this hop took 70 milliseconds. This one took 73. This one down here took 82 milliseconds on average to respond. By doing this we have built a map of hops between this system where
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