RFC 1918 addresses

Paul A Vixie paul at vix.com
Sun Jun 1 04:33:11 UTC 1997

> While this is technically correct, actually changing the addresses in ICMP
> payloads violates my first rule of debugging complex systems:
> 	Diagnostics should be a simple as possible and never be altered by any
> non-diagnostic system.

Except in the case of ICMP's and L3 NAT, I agree with this.  Correctness is
also important.

> (Which begs the question of should people be using these addresses locally
> at all.  Which is kind of metaphysical except that it *does* preserve
> address space, which is a universal good.  Some people do this for security
> reasons too, although it's at best only moderately effective security
> measure and could produce a false sense of security inside such an
> addressing realm.)

RFC 1597 was a great thing.  RFC 1918's language wasn't as strong as I would
have liked (that is, I liked 1597's applicability statement better) but it
names the same nets and thus I approve heartily of it.  Private networks are
a good thing.  But they have to be private, and as shown by the ICMP examples
as well as the PMTUD reference earlier in this thread, transit links are not
private.  So, use private addresses for leaf nets (even multiply homed ones
if you can find a NAT solution that will do that for you), not transit nets
like ISP T1's.

> I agree that ever having a source or destination IP that's RFC1918 outside
> the domain is a very bad thing.   

I don't see anyone here disagreeing with that, but apparently a number of
ISP's did not consider the ICMP case when they gave numbers to their T1's,
and so it's a question of definition rather than of intent.  Transit nets
are public, not private, and so they have to have public, not private,

Yikes.  A technical discussion on NANOG.  What's the world coming to?

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