Testing Routability

If you can ping your default router but you cannot ping a given host on the Internet, something is wrong with one of the routers between your network and the destination host. To find out where exactly it goes wrong, you can use the traceroute command. The traceroute command uses the Time To Live value of the UDP datagrams it sends. The idea is that when the TTL reaches zero, a datagram is discarded by the router that discards the packet, and a message is sent to the sender. When starting, traceroute uses the TTL 0, which causes the packet to be discarded already by the first router. This allows traceroute to identify the first router. Next, it sends the packet to the target destination again, but with a TTL of 1. This causes the packet to be discarded by the second router. traceroute goes on this way until it reaches the final destination.

To use traceroute, you usually put the host name as the argument. For example, you'd use traceroute www.novell.com. It is, however, possible to put in the IP address of a host. The result will look like Figure 13-14.

File Edit View Terminal Ji b'

laksmi:/ # traceroute www.novell.cora

traceroute to www.novell.com (130.57.5.25), 30 hops max, 40 byte packets

1 192.168.1.254 (192.168.1.254) 25.075 ras 18.738 ms 16.133 ras

2 195.190.249.90 (195.190.249.90) 44.555 ras 46.594 ms 16.527 ras

3 42.ge-4-0-0.xrl.3dl2.xs4all.net (194.109.5.49) 25.971 ms 18.744 ms 1 8.310 ms

4 asd-dc2-ias-url0.nl.kpn.net (194.151.244.74) 21.964 ras 19.623 ms 15. 754 ms

7 asd2-rou-1021.NL.eurorings.net (134.222.230.78) 38.238 ras 33.665 ras 30.715 ms

8 nyk-sl-rou-1001.US.eurorings.net (134.222.231.230) 116.567 ms 107.140 ras 106.014 ras

9 nyk-sl-rou-1003.US.eurorings.net (134.222.230.98) 116.863 ms 125.124 m 123.473 ms

10 marl-pos-l-0.newYorknyd.sawis.net <208.173.134.69) 118.261 ras 115.119 ms 112.258 ras

11 bcs2-so-3-2-0.NewYork.sawis.net (204.70.193.30) 108.618 ms 105.405 ms 102.385 ms

12 bcs2-so-4-0-0.Washington.sawis.net (204.70.192.1) 113.547 ms 123.805 ms 115.693 ms

13 bcsl-so-7-0-0.Washington.sawis.net (204.70.192.33) 111.401 ms 106.096 ms 101.562 ras

14 dcrl-so-3-0-0.Atlanta.sawis.net (204.70.192.53) 110.257 ms 106.234 ras 102.050 ms

15 dcrl-so-l-3-0.dallas.sawis.net (204.70.192.78) 141.943 ms 150.728 ras

Figure 13-14. The traceroute command shows router for router the exact route that is used to a given destination.

In the traceroute command, you will see every router that is passed. For each router, the name of the router is displayed, followed by the IP address of that router, which is followed by the round-trip times of the three packets that were sent to that router.

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