The traceroute command does this by sending a number of echo request packets (like ping does) but with varying time-to-live (TTL) settings so that it can calculate the time that each hop requires. For example, for the first hop, the TTL is set to 1. For the second hop, it's set to 2, etc.
$ traceroute world.std.com
traceroute to world.std.com (192.74.137.5), 30 hops max, 40 byte packets
1 * * *
2 gig0-8.umcp-core.net.ums.edu (136.160.255.33) 2.634 ms 2.632 ms 2.610 ms
3 ten2-0.stpaul-core.net.ums.edu (136.160.255.198) 3.515 ms 3.508 ms 3.486 ms
4 te4-3.ccr01.bwi01.atlas.cogentco.com (38.104.12.17) 4.169 ms 4.163 ms 4.143
ms
5 te4-2.ccr01.phl01.atlas.cogentco.com (154.54.2.174) 6.268 ms 6.262 ms
te3-3.ccr01.phl01.atlas.cogentco.com (154.54.83.221) 6.950 ms
6 te0-0-0-19.mpd21.jfk02.atlas.cogentco.com (154.54.2.110) 9.835 ms
te0-0-0-7.ccr22.jfk02.atlas.cogentco.com (154.54.31.53) 8.937 ms 8.925 ms
7 te0-1-0-4.ccr22.bos01.atlas.cogentco.com (154.54.6.9) 14.768 ms
te0-2-0-6.ccr22.bos01.atlas.cogentco.com (154.54.44.58) 14.129 ms te0-1-0-
2.ccr21.bos01.atlas.cogentco.com (154.54.44.6) 14.740 ms
8 te4-1.mag01.bos01.atlas.cogentco.com (154.54.43.50) 14.450 ms
te7-1.mag02.bos01.atlas.cogentco.com (154.54.7.42) 13.859 ms
te4-1.mag01.bos01.atlas.cogentco.com
(154.54.43.50) 14.816 ms
9 vl3884.na31.b000502-0.bos01.atlas.cogentco.com (38.20.55.82) 18.336 ms 16.398
ms 16.699 ms
10 cogent.bos.ma.towerstream.com (38.104.186.82) 13.925 ms 13.840 ms 13.720 ms
11 g6-2.cr.bos1.ma.towerstream.com (64.119.143.81) 21.495 ms 15.647 ms 15.458 ms
12 69.38.149.18 (69.38.149.18) 33.680 ms 33.602 ms 33.419 ms
13 64.119.137.154 (64.119.137.154) 31.961 ms 30.079 ms *
14 world.std.com (192.74.137.5) 34.695 ms 34.698 ms 34.159 ms
The ping command is popularly used to test connectivity with a remote system and verifies that you can (or can't) reach the remote system.
Route Caching
The route -Cn command displays routing cache information. This shows routes associated with active connections. Linux caches this information so that it can route packets faster.
route -Cn Kernel IP routing cache Source Destination Gateway Flags Metric Ref Use Iface 192.168.0.3 192.168.0.6 192.168.0.6 il 0 0 13 lo 192.168.0.6 204.111.97.254 192.168.0.1 0 0 0 eth0 192.168.0.6 204.111.97.254 192.168.0.1 0 2 0 eth0 192.168.0.6 204.111.97.254 192.168.0.1 0 0 4 eth0 192.168.0.6 192.168.0.3 192.168.0.3 0 1 0 eth0 204.111.97.254 192.168.0.6 192.168.0.6 l 0 0 79 lo
Rejecting connections
You can also specifically reject specific network connections using route commands.
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