Private use of non-RFC1918 IP space

Matlock, Kenneth L MatlockK at
Mon Feb 2 19:01:30 UTC 2009

I see 2 problems off the top of my head with using public IP blocks for
private networks.

1) You're not going to be able to reach servers/services/etc that
actually have allocated those IP blocks. (May or may not affect you, but
that's your issue to deal with in the future).
2) (and more important) It really makes it easy to 'accidentally'
announce that public IP block out in the future, unless you have proper
announce filters in place (And if something as basic as subnetting isn't
done properly, I doubt route filtering is either). This one not only
affects you, but affects the netblock that gets mistakenly announced

RFC1918 space was designed to prevent these issues.

Ken Matlock
Network Analyst
Exempla Healthcare
(303) 467-4671
matlockk at

-----Original Message-----
From: sthaug at [mailto:sthaug at] 
Sent: Monday, February 02, 2009 11:56 AM
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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