Re: 192.168.x.x oddities

["steve" <securityfocus () delahunty com>]

I'll hazard a guess that your ISP is also using the 192.168.x.x IP range to
provide IPs to their cable modems.  Your router is likely something like
192.168.20.1 or something and then your cable modem is 192.168.100.123 and
that is on your ISPs network.

When you are pinging that IP you find that you think is outside your
network, you are likely right that it is at the ISP but that is still not
the Internet proper.

If you do a trace route (tracert) to some website like securityfocus.com you
might see your IP, your router's IP, your cable modem IP, then IPs on your
ISPs network, then real ones.

So probably don't fear that someone is lurking on your own home network.

Hope this helps.


----- Original Message ----- 
From: "Jimmy Brokaw" <hedgie () hedgie com>
To: <security-basics () securityfocus com>
Sent: Monday, June 14, 2004 5:48 PM
Subject: 192.168.x.x oddities


This seems like a stupid question from a non-guru like me, but I've asked
a couple of the "gurus" I know and gotten nothing but strange looks.

I run a small network at home, using a wireless router to connect to a
cable modem.  My internal IPs all fall in the 192.168.0.x range, which is
the only address-space the router is configured to support.  I've got
authentication and logging, so before anyone says "I bet it's a neighbor
using your connection," I've verified nobody else is logging in.

My understanding is that the entire 192.168.x.x range is for internal
networks only (RFC 1918), and unrouteable on the Internet.  When I run the
following command, however, I can see several computers:

[computer]$ nmap 192.168.*.* -sP

I get what looks like four computers (in addition to mine), plus some x.0
and x.255 addresses responding to the pings.  I picked one at random, and
it appears to belong to my ISP.  Doing a traceroute, I found the packet
reached its destination at a public (routeable) address, indicating to me
the machine has two addresses on the same interface.  RFC 1918 states:

   One might be tempted to have both public and private addresses on the
   same physical medium. While this is possible, there are pitfalls to
   such a design (note that the pitfalls have nothing to do with the use
   of private addresses, but are due to the presence of multiple IP
   subnets on a common Data Link subnetwork).  We advise caution when
   proceeding in this area.

Am I therefore correct in my assumption that the ISP is routing my pings
onto their internal network?  Is this a normal response?  It seems like
there ought to be security concerns here, but I can't nail them down,
except the assumption that traffic destined for 192.168.x.x addresses may
not be filtered as well (or at all), since it may be assumed it originated
from within the internal network.




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Ethical Hacking at the InfoSec Institute. Mention this ad and get $545 off 
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to facilitate one-on-one interaction with one of our expert instructors. 
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