Companies using public IP space owned by others for internal routing

Narseo Vallina Rodriguez narseo at
Mon Dec 18 14:22:50 CST 2017

We found a number of such instances when working in our last year's
Internet Measurements Conference (IMC) paper [1] "A Multi-perspective
Analysis of Carrier-Grade NAT Deployment".

Back then, spring-summer 2016, we found a number of large cellular
ISPs using routable IP address space for their private IP address

Some cases were AS3651 (Sprint US) AS22140 (T−Mobile US) and AS24608
(H3G SpA IT) among others.

We found this practice to be common in other ISPs but we did not have
enough data to drive anything conclusive for them.

One of the most common blocks was assigned to the UK Ministry of
Defense as Shaun also pointed out for Microsoft.

You can find the whole discussion is in Section 6.1 and Figure 7.


On Mon, Dec 18, 2017 at 2:58 PM, Jason Iannone <jason.iannone at> wrote:
> My previous employer used 198.18/15 for CE links on IPVPN services.
> Walgreens used an American SP's space internally and couldn't talk to
> any users in that space as a result.
> On Sun, Dec 17, 2017 at 11:31 PM, Eric Kuhnke <eric.kuhnke at> wrote:
>> some fun examples of the size of ipv6:
>> On Sun, Dec 17, 2017 at 7:05 PM, Large Hadron Collider <
>> large.hadron.collider at> wrote:
>>> Missent.
>>> Welcome to IPv6, where you have technically-reserved-for-future-use space
>>> that should never actually need to be used. Quite likely, you can use
>>> something like 440::/16 as your private space, but please don't do that
>>> unless you've exhausted the true private space.
>>> You're welcome.
>>> On 17/12/2017 14:57, James Downs wrote:
>>>> On Dec 17, 2017, at 14:33, Matt Hoppes <mattlists at>
>>>>> wrote:
>>>>> Had a previous employee or I discovered it on the network segment after
>>>>> we had some weird routing issues and had to get that cleaned up. I don't
>>>>> know why anyone would do that when there is tons of private IP space.
>>>> Unless there isn't.. I've worked at more than one company that had used
>>>> up all the private space. Then you have the cases where some M&A causes
>>>> overlapping IP space. In addition, you'd also be surprised how many people
>>>> just assign the entire 10/8 space into a flat IP space.
>>>> -j


Narseo Vallina-Rodriguez
Research Staff, ICSI
Personal web:

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