IPv4 Address Space Utilisation

I read earlier another warning from the EC that businesses should be looking to deploy IPv6 as soon as possible. We know, of course, that this is important: IANA ran out of /8 blocks almost a year ago, and APNIC’s allocation ran dry soon after. The others will follow suit soon. But how much of that is being used?

We are nearing 400,000 prefixes in the IPv4 BGP default-free zone. 398,135 are visible to route-views2 in its latest table dump at time of writing. But the allocated address space is not indicative of actual utilisation, nor is the number of prefixes being held in full tables. When it comes to utilisation, it’s the amount of address space advertised by those prefixes that is more interesting.

The IPv4 address space is primarily used for unicast addressing. IPv4 offers 32 bits per address, allowing 4,294,967,296 (232) potential addresses. Some blocks of this space are marked for particular uses and cannot be routed globally. These blocks are:

That accounts for 588,383,488 addresses that are not globally routeable IPv4 space, leaving 3,706,583,808 from the total.

These are covered in more detail in RFC 5735. Get that “Future Use” block out of your mind right now; those sixteen /8s sure would be useful for a short time, but they’re effectively out of bounds. We don’t know how much hardware explicitly drops packets purporting to have come from, or drops packets destined for, this range.

To reach 100% utilitisation, these ~3.7 billion addresses would all have to be lit up, but we’re not quite there yet. Counting only the shortest prefixes advertised today that fully cover the advertised address space, there are 2,497,743,276 publicly routeable IPv4 addresses out there. (To be clear, since multiple small blocks from a parent block can be advertised over BGP, I count only the shortest prefixes and ignore the overlapping space beneath. Also, I’m simplifying and not discounting special addresses such as broadcast addresses.)

That is, today we’re at about 67.4% utilisation. The Internet is two-thirds full. The last time I checked this, perhaps 2 years ago, we were closer to 50%. I should draw a graph.

Of course, I should make clear that I am not suggesting we should be aiming for full utilisation. Equally, “utilisation” here doesn’t indicate actual use; much of the space advertised is possibly unused, suggesting more efficient packing could take place. But it’s certainly utilised in the sense that it cannot be used by anybody else.


Posted by Stephen Strowes on Tuesday, December 6th, 2011. You can follow me on twitter.

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