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In a LAN if I traceroute the IP address of any other machine, it gets it in one hop <1 ms.

However, there is one machine, which if I traceroute, it exits the private network, goes out through external IP adresses, until it stops because of a timeout.

A mail server runs on that machine and it's working properly, if this information is of any help.

What can be the cause of such a strange behavior?

Aktive Routen:
     Netzwerkziel    Netzwerkmaske          Gateway   Schnittstelle  Anzahl
       20       1       20       20       20       20       1
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Please show us traceroute output and routing table of host from which you're tracerouting. – Gevial Apr 9 '13 at 6:51
I don't quite understand it. Is that information really absolutely necessary to answer the question? I was expecting something like it is completely normal and can happen due to <short explanation> or something like this shouldn't normally happen. Please tell me why such personal information is necessary, like my MAC Adress, or my network provider (which is visible in the tracert output) and such my current location, external IP Adress, etc. I can, however, post the route list of my routing table if you find it useful. – vsz Apr 9 '13 at 8:06
You can replace personal info with xxx. Commands output may lit a little light on the problem. – Gevial Apr 9 '13 at 8:18
up vote 2 down vote accepted

Simple. Due to the routing it falls back to the default route, unless 192.168.x.x is 192.168.1.x - there is a specific route only for the 192.168.1

Anything else (i.e. will go via default route and that may leave the network. Any non-stupid admin will filter those IP addresses on the gateway router, for security reasons.

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SOHO-grade CPE often doesn't have those filtering capabilities. – Alnitak Apr 9 '13 at 8:19
Accepted answer. And I'm an idiot because of not thinking of this possibility. (I'm not an admin, just a curious user) – vsz Apr 9 '13 at 8:21
@Alnitak and that is how relevant? I mean "soho grade CPE" are something that people talk on SuperUser - not where things are done in a professional capacity (serverfault) ;) – TomTom Apr 9 '13 at 11:37
@TomTom there's professional, and there's using RFC 1918 space and not knowing how routing works... – Alnitak Apr 9 '13 at 12:16
@Alnitak no, you mentioned SOHO grade CPE. No professional will use that in a network. A professional will know for example MIkrotik CPE that are dead cheap and can do quite everything. – TomTom Apr 9 '13 at 13:49

A traceroute (and indeed any other packet) should only leave the local network if the address in question is not in the local network.

If the destination address is in the same subnet as your machine's own interface then the system should use ARP to determine the relevant MAC address, and then send the packet there directly.

Your routing table output shows you are using on your local LAN. Is the address in question perhaps on some other portion of ?

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Other possibility might be a site2site VPN...


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