Private use of non-RFC1918 IP space

TSG tglassey at
Mon Feb 2 16:21:45 UTC 2009

Joe Greco wrote:
>> On Feb 2, 2009, at 10:57 AM, Jeffrey Ollie wrote:
>>> On Mon, Feb 2, 2009 at 9:48 AM, Trey Darley <trey at>  
>>> 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.
>>> I'd recommend against it, because even though the network is not
>>> connected to the Internet now you never know what the future holds.
>>> Even if it's never connected there are always things that seem to pop
>>> up and cause problems.
>>> Also, if you're address allocation policy has been so badly managed
>>> that you've run out of space in adding more IPs to the pool
>>> isn't going to help for very long.
>> It will if you manage it better.
>> Fortunately, there's a /12 and a /24 still left.
> And a /16.  (What's the /24?)  And possibly some other space that is
> reserved-for-other-purposes.
>> A /12 is more space  
>> than 99.99% of the networks on the Internet need, so why wouldn't that  
>> suffice instead of using "real" space.
> If you absolutely, positively *had* to allocate another /8, it'd probably
> be best to look through Class A space for networks that are not likely to
> ever appear on the Internet.  ISTR a bunch of them are assigned to the US
> military, for example.
> ... JG
For which the unauthorized use of could be construed as a military 
attack if those pirated addresses ever appear on the open Internet from 
this ISP... No that's also a really bad idea. I find it really 
troublesome to believe that the subnetting on a site was so complex that 
it ate an entire /8. What I am betting is that for some reason that ISP 
wants its addressing to be totally flat and not replicated.


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