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Re: Fwd: 41/8 announcement

  • From: Stephen Sprunk
  • Date: Fri May 26 16:13:58 2006

Thus spake "william(at)" <[email protected]>
On Fri, 26 May 2006, Bill Woodcock wrote:
Presumably they're double-natting.  I had to do that once for Y2K
compliance for three large governmental networks that were all statically
addressed in net-10 and wouldn't/couldn't renumber in time.  In fact,
there were _specific hosts_ which had the same IP address, and _had to
talk to each other_.  Gross.  But it can be done.
Please explain how. I simply can't imagine my computer communicating
with another one with exactly same ip address - the packet would never
leave it. The only way I see to achieve this is to have dns resolver
on the fly convert remote addresses from same network into some other
network and then NAT from those other addresses.
Unfortunately, I've done this several times, most notably within one company that had multiple instances of 10/8 that needed to talk to each other. A decent (if one can use that term) NAT device will translate the addresses in DNS responses, so two hosts that both live at will see the other's address as, for example,, both in DNS and in the IP headers.

It's extremely ugly, but that's what one gets for using private address space. This exact scenario was a large part of why I supported ULAs for IPv6.


Stephen Sprunk "Stupid people surround themselves with smart
CCIE #3723 people. Smart people surround themselves with
K5SSS smart people who disagree with them." --Aaron Sorkin