private ip addresses from ISP

Brian Johnson bjohnson at
Tue May 23 15:22:35 UTC 2006


> -----Original Message-----
> From: owner-nanog at [mailto:owner-nanog at] On 
> Behalf Of Joe Maimon
> Sent: Tuesday, May 23, 2006 10:15 AM
> To: Robert Bonomi
> Cc: nanog at
> Subject: Re: private ip addresses from ISP
> Robert Bonomi wrote:
> > 
> > TTL-E messages _do_ have legitimate function in network management.
> > TTL-E messages _can_ originate from RFC1918 space,  
> addressed to 'public
> > internet' addresses.  Usefully, and meaningfully.  Ever 
> hear of 'traceroute'?
> > Ever use it where packets went across a network using 
> RFC1918 internally?
> > Ever had a route die _between_ two RFC1918 addressed nodes 
> on somebody elses
> > network?
> I guess this means that providers who utilize rfc1918 along 
> their hops 
> should make an effort to ensure these addresses are not used for icmp 
> messages or translate these addresses when they source icmp.
> Understandably, translation on providers networks is not 
> always feasible.
> A feature on routers that sourced icmp packets to be told specificaly 
> which address of the router to source it from would also help.

In the Cisco world, I thought that the source would always be the interface
that replies to the ICMP packet. That seems to be good form to me.

Where am I going wrong?


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