Re: Traceroute UDP filtering???

From: Chris (Chris@nospam.com)
Date: 01/20/03 

From: "Chris" <Chris@nospam.com>
Date: Mon, 20 Jan 2003 17:27:52 -0000

"Mike Bueide" <mbueide@charter.net> wrote in message
news:v2ob1isup3h9f6@corp.supernews.com...
> Chris wrote:
> >>I have a similar query. I have a cisco broadband
> >>router connected to a cable modem (my ISP is
> >>charter.) I'm using the dhcp client on the wan
> >>interface and nat to the lan interface. I do get
> >>a public address assigned to my wan interface, but
> >>if I do a traceroute from inside my lan to the
> >>internet, I also find there is a public ip address
> >>on the second hop and no mention of the WAN ip
> >>that was assigned? Does anybody have an answer?
> >>
> >>Here a copy of my traceroute.
> >>1 c806 (192.168.1.17) 1.620 ms 1.382 ms 1.415 ms
> >> 2 10.200.0.1 (10.200.0.1) 9.891 ms 8.458 ms
> >>9.437 ms
> >> 3 12.125.144.25 (12.125.144.25) 39.685 ms
> >>39.365 ms 28.956 ms
> >> 4 gbr2-p57.dvmco.ip.att.net (12.123.36.110)
> >>27.870 ms 31.975 ms 33.458 ms
> >> 5 gbr4-p80.dvmco.ip.att.net (12.122.5.29)
> >>36.857 ms 54.521 ms 33.887 ms
> >> 6 gbr4-p80.dlstx.ip.att.net (12.122.2.101)
> >>63.573 ms 56.278 ms 47.910 ms
> >> 7 gbr5-p70.dlstx.ip.att.net (12.122.5.77)
> >>56.629 ms 52.926 ms 85.692 ms
> >> 8 *^X^C
> >>
> >>
> >>Note the 10.200.0.1 ip.
> >>
> >
> >
> > Using private RFC1918 addresses is becoming more common in ISP's. We use
> > various 10.x.x.x ranges within our network to save on precious real
> > addresses which we are trying to save. They don't have to be internet
> > routeable as long as they are routeable on the ISP's network and
outgoing
> > packets have an internet routeable source address.
> >
> >
>
> So then the cable modem is actually assigned the
> 10.200.0.1 private ip? Is the public ip packet
> simply encapsulated and delivered to the modem
> which in turn takes the wrapper off and delivers
> the internet packet with the public ip to the
> routers wan interface?
>
> The reason I ask this is because I had the great
> idea of blocking all private ip addresses from my
> wan interface and found that dhcp no longer
> worked. I had to remove the acl referencing the
> 10.0.0.0 block to get my dhcp back up.
>

You would have to look at your router to see what IP address you have and if
it appears on a trace. You should have a public IP assigned somewhere to
enable internet traffic to reach you but your ISP can still use private
ranges within their network.

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