Re: RE:Website published on ADSL

[Ant Skelton <ant@xxxxxxx>]

Paul Gordon wrote:
>
> Hmmm, I guess that could be so... I find it quite bizzarre that BT
would route
> traffic in that way though - doesn't it seem a bit "odd" to
you?

Not really, they're quite entitled to use private IPs for their network.
To use public routable IP addresses for these hosts would be wasteful.

> Surely the TTL should not expire in this case, - isn't the default TTL
value
> 30?

I think a quick explanation of how traceroute works is in order!
Traceroute starts by sending a probe packet with a TTL of one. The TTL
is decremented by the first router that the packet traverses. The router
notices this, and sends a TIME_EXCEEDED message back. Traceroute now
knows the first hop.

Next, it sends a packet with TTL of 2, which elicits a reply from the
second router on the path. It continues with incrementing TTLs until
either the target host is reached or some TTL limit (default 30,
settable with -m on unix or -h on 'doze) is reached.

The first column of traceroute output is, in fact, the TTL for that
packet. Traceroute actually sends a number of packets for each TTL value
(default is 3, settable with -q on unix), which is why you get more than
one column of timing info for each TTL.

So, looking at one of the contentious lines of traceroute output:

> > > 13   975 ms  1139 ms  1099 ms  172.16.93.126

Three probes were sent with TTL=13, and their TTL hit zero at a router
with IP address 172.16.93.126.

> BTW, sorry about the plaster dust in that PC - I left the loft hatch
open
> when I was having the hall/stairs/landing replastered!! - everything
up
> there is COVERED. (still, at least I got plenty of cables in the wall
before
> the plaster went up  :-)

Heh, no worries ;). That particular PC is now contributing parts to
various homebrew projects!


cheers

ant
ant@xxxxxxx

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