Private use of non-RFC1918 IP space

Stephen Sprunk stephen at
Mon Feb 2 20:28:33 UTC 2009

Trey Darley wrote:
> Some colleagues and I are running into a bit of a problem. We've been using RFC 1918 Class A space but due to the way subnets have been allocated we are pondering the use of public IP space. As the network in question is strictly closed I don't anticipate any problems with this as the addresses would be unambiguous within our environment. I'm curious if anyone else is doing this.

"Closed" networks nearly always end up getting connected to public 
networks, either by intent or by accident.  If you act as if your 
network will remain "closed" forever, e.g. by using public addresses 
that are (or will be) assigned to someone else, you're going to cause a 
lot of headaches for yourself or your replacement down the road, eventually.

Contrary to popular belief, ARIN (and possibly other RIRs) _will_ assign 
public IPs for private/closed networks if you can explain why RFC1918 
space will not suffice for your needs, e.g. because you are running a 
private internetwork between multiple companies and thus NAT/RFC1918 is 
simply not viable due to the number of ASes and the difficulty in 
avoiding collisions or the sheer number of hosts...


Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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