Destination show up twice in traceroute

[Brian Chabot]

bscott at ntisys.com wrote:

>>I'm wondering why a machine at work shows up twice on the traceroute
>>output (see below) when I do the trace route from home.

[snip great explanation]

> I could also imagine some brain-damaged NAT
> implementation or firewall somewhere causing trouble.

It doesn't even have to be completely brain damaged.  If you traceroute 
to an  IP address on my DSL line, you will see the same result.

This is because I'm not "routing" my IP block.  I'm using NAT instead to 
port forward ALL ports to an internal IP address.  This has the result 
of both the router and the internal machine answerring to the ICMP 
request and claiming to be the same IP address.  The IP is routed to but 
not through the router and thus the router answers as the IP if the TTL 
is too short to pass it to the port on the internal network.  When the 
TTL is increased, the actual internal machine gets it and answers as 
it's internal IP which the router NAT's as the external IP.

So why would I do this sort of bass-ackward network setup?  Well, by not 
routing the IP block, the entire block becomes usable.  This frees up 
the network address, the router's address, and the broadcast IP for the 
block.  Hence, in a block of 8 IP addresses where normally only 5 are 
usable, I now have all 8 usable.  This usually confuses the hell out of 
tech support people even in a usually knowledgable ISP like Speakeasy.

When you have a limited number of addresses and need every one you can 
get, this makes sense.

It may not be the answer in your case but it is an answer that is at 
least true on my network.

Brian
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