Private use of non-RFC1918 IP space

Paul Stewart pstewart at
Mon Feb 2 19:01:29 UTC 2009

Yeah, agreed....  I had a customer last week call us because we were
"blocking them from an important site".  After someone called them they
found we could access the website no problem... upon further
investigation we found their internal IP space had been numbered as

When we asked them why this IP was used, they told us that a $200/hour
network consultant had upgraded their network last week with new Windows
servers, new router, new etc. etc. etc.... and that this new IP
numbering "sounded like a good number"...

Politely told them they were paying too much and next time call someone
who knows what they are doing.... "you got ripped off, sorry about your

Oh, and their internal IP space = ;)


-----Original Message-----
From: sthaug at [mailto:sthaug at] 
Sent: February 2, 2009 1:56 PM
To: darcy at
Cc: nanog at
Subject: Re: Private use of non-RFC1918 IP space

> > Company A uses public IP block A internally. Company B uses public
> OK, so we start out with a bad network design then.

No. We start with blocks A and B which are both properly allocated by
the relevant addressing authorities.

> > block B internally. Company A and B later merge, and connect their
> > networks. No conflict, no renumbering needed (at least not right
> Maybe.  What if they both happened to choose  Is this just
> matter of decreasing the odds of a conflict?  It still seems like bad
> network management to me.

My assumption throughout this whole discussion, which clearly has not
been understood, is that the public IP block used internally is a
properly allocated by the relevant addressing authority. That is, for
me, the whole point of using public addresses to guarantee uniqueness.

Steinar Haug, Nethelp consulting, sthaug at



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