Re: RE:Website published on ADSL

[Ant Skelton <ant@xxxxxxx>]

Paul Gordon wrote:
>

> 12   741 ms   701 ms   701 ms  213.120.207.222
> 13   975 ms  1139 ms  1099 ms  172.16.93.126
> 14  1126 ms   962 ms   865 ms  172.16.93.53
> 15   961 ms   837 ms   810 ms  172.16.100.57
> 16   783 ms   988 ms   879 ms  172.16.109.5
> 17   920 ms   426 ms   412 ms  62.7.250.131
> 18     *     1016 ms  1139 ms  host213-120-145-98.btopenworld.com

> Now, what concerns me is the hops numbered 13 through 16 - the
172.16.X.X
> ones. These are in the non-routable private address space as defined
by
> RFC1597 and CANNOT be on the internet.

My guess is that these are transit routers within BT's infrastructure.
The 213.120.207.222 is the border router which connects the internet to
BT's network.

You're right, they're private IPs, and as such you can't ping them
directly, because they're non-routable. However, there's nothing to stop
them seeing you. When you traceroute to 213.120.145.98 and the packet's
TTL expires at one of these 172.16 routers, it throws your packet away
and replies with an ICMP TIME_EXCEEDED message, where the *source* IP
address is the router's 172.16.* address and the destination address is
your IP address, which is routable, hence the packet gets back to you.

If one of these routers were attempting to send any packets to you for
which a reply might be reasonably expected (for example a TCP SYN), you
wouldn't see a 172.16 address, because the packet would have to be NAT'd
in order that the router might see a reply - for example, you'd probably
see the packet as having a source IP address 213.120.207.222 - NAT'd by
that border router.

However, since an ICMP TIME_EXCEEDED requires no reply, it would be a
waste of time to NAT the packet, and besides, that would lead to a
misleading picture of the network (although I've seen plenty of NAT code
that performs NAT on all ICMP packets).

If you try traceroute to another 213.120.145.* address, which presumably
are other people's DSL IP addresses assigned by BT, you'll see a similar
trace.

cheers

ant
ant@xxxxxxx

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