Fwd: 41/8 announcement

Stephen Sprunk stephen at sprunk.org
Fri May 26 20:11:37 UTC 2006

Thus spake "william(at)elan.net" <william at elan.net>
> 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 


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 

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