Thirty percent of IPv4 space is still unused

Based on calculations that I have recently done, nearly a third of IPv4 address space is not in active use. The IP address calculations refer to consecutive IP blocks not advertised by any of the autonomous systems on the global Internet routing tables, not to individual addresses in existing subnets. This amount seems large and has potentially interesting consequences.

Here is how the math was done: using a script, the IPv4 address space was enumerated, and for each IP address, an ASN lookup is performed using data provided by Route Views.  Putting aside the private, multicast and other reserved ranges, I found that out of approximately 14.5 million /24 address blocks, 4.4 million are ‘unrouted’, i.e., not assigned to any autonomous system in BGP routing tables. This amounts to 30.3 percent. The BGP data is from July 30th, 2012.  Mapping the unrouted blocks to the IANA address registry, we see that two thirds of the unrouted blocks are in the so-called legacy space.

What makes this figure interesting is that the largest RIRs have announced that their pool of unallocated IPv4 addresses is either exhausted (APNIC) or near exhaustion (ARIN and RIPE-NCC). My calculations show that the lack of unallocated addresses from the RIRs is not quite the same as IPv4 addresses having run out!

What are the implications of the more than one billion allocated yet unused IPv4 addresses? It points to a potentially lucrative IPv4 transfer market for the next few years. We have a resource that has a certain degree of scarcity, yet is in adequate supply to be tradable, and many potential buyers and sellers. This makes the need for transparency and design of efficient market mechanisms more apparent, and could affect the economics of IPv6 deployment in the coming years. (See Geoff Huston’s recent piece for a discussion of depletion rates in relation to IPv6 adoption.)

Written by Hadi Asghari, Delft University of Technology.


  1. andrewdul

    Just because address space is not announced does not mean it is unused completely.  While routing IPv4 addresses on the “Internet” is the most common use of IP address space, it is not the only place that it is used.  I would agree there is some difference between allocated and used, but this is hardly new information to the network operator community. 

  2. jcurranarin

    Hadi – 
    Your inference regarding the significant amount of IPv4 address space unannounced is almost certainly correct, i.e. “It points to a potentially lucrative IPv4 transfer market for the next few years. We have a resource that has a certain degree of scarcity, yet is in adequate supply to be tradable, and many potential buyers and sellers. ”   As has been already noted, unannounced doesn’t equate to unused but it still quite reasonable to assume that some amount of this unannounced space could become “available for transfer” depending on market price.  
    This is desirable feature of the IP number resource system, as not all Internet service providers are at the same point in their adoption of IPv6, and having some amount of underutilized IPv4 space provides a temporary “fix” for those service providers who might otherwise have none.  Even with the significant boost from World IPv6 Launch Day (where major websites including Google, Facebook, Youtube, Yahoo, Bing, etc made their websites permanently via IPv6), the movement of Internet content to IPv6 is going to take time, and hence some availability of IPv4 address space for compatibility gateways is desirable.  Some providers will use this opportunity to extend their current IPv4-based services, and some will use it to shift to IPv6-based services with access to legacy IPv4 content.   These approaches are quite similar, and both benefit from access to underutilized IPv4 address space.
    The main difference between them is that relying on just IPv4 results in ever increasing costs due to increasing scarcity, whereas connecting customers with IPv6 (with dynamic IPv4 address usage as needed) actually becomes easier as more more content is accessible via IPv6.  This is likely why you see providers such as T-Mobile, Verizon, Comcast and others making major commitments to transition as soon as possible.
    Thanks for your analysis in this crucial area!
    John Curran
    President and CEO

  3. McTim

    geode morgen,
    In addition to what JC and Andrew have said, your use of the word “exhaustion” may be misleading.
    APNIC has reached it’s exhaustion phase, which means it gives out smaller blocks by default.  It has not yet “run out” of IPv4 addresses completely IIUC.