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Consider:

 - saturn:~$ mtr -rc 5 nuance.com
HOST: saturn                      Loss%   Snt   Last   Avg  Best  Wrst StDev
  1.|-- dir-320                    0.0%     5    1.7   0.7   0.4   1.7   0.6
  2.|-- ???                       100.0     5    0.0   0.0   0.0   0.0   0.0
  3.|-- bzq-218-227-242.red.bezeq  0.0%     5   10.1  10.4   9.7  10.7   0.4
  4.|-- bzq-179-75-165.static.bez  0.0%     5   10.9  10.3   9.8  10.9   0.4
  5.|-- bzq-179-124-193.static.be  0.0%     5   10.8  10.6  10.3  10.8   0.2
  6.|-- bzq-179-124-230.static.be  0.0%     5   11.0  11.0  10.6  11.4   0.3
  7.|-- 213.242.116.129            0.0%     5   69.6  69.8  69.6  70.3   0.3
  8.|-- ae-0-11.bar1.Marseille1.L  0.0%     5   64.8  64.7  64.4  65.2   0.3
  9.|-- ae-7-7.ebr1.Paris1.Level3  0.0%     5   64.1  64.7  64.1  65.3   0.4
 10.|-- ae-59-114.csw1.Paris1.Lev  0.0%     5   80.1  74.0  70.9  80.1   4.2
 11.|-- ae-1-51.edge4.Paris1.Leve  0.0%     5   64.4  67.7  64.4  80.5   7.1
 12.|-- global-crossing-xe-level3  0.0%     5   64.6  64.7  64.5  65.2   0.3
 13.|-- NUANCE-DICTAPHONE.ge-3-0-  0.0%     5  142.5 165.8 142.5 257.9  51.5
 14.|-- www.nuance.com            40.0%     5  144.3 144.0 143.7 144.3   0.3

The dir-320 node is my router (wired), and I suspect that the ??? node is the ADSL modem.

Considering that the modem has 100% packet loss, how does anything get through at all? Are the packets going through but the machine just not acknowledging them?

Thanks.

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up vote 9 down vote accepted

It's not that the machine loses packets, it's that it didn't return that packet. A traceroute works by limiting the TTL on a UDP (most OSes' traceroute) or ICMP (in the case of Windows tracert) packet, and if a machine is configured to not return the ICMP Time Exceeded message, your machine will see this as packet loss. Otherwise the machine will function as normal.

http://en.wikipedia.org/wiki/Traceroute#Implementation

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