IPv4 is a finite resource, but IPv6 is huge. The length of an IPv6 address is 128 bits, compared with 32 bits in IPv4. In theory, this means an almost endless number of combinations can be created, and the same cautious IPv4 thinking doesn’t apply in an IPv6 world. However, that doesn’t mean you shouldn’t keep track of your IPv6 resources. IP planners and network architects still need to make some sense of what resources are in use.
Well, once IP space is deployed, you don’t want to move it. In global addressing space for IPv6, there is no way to overlap space as there is in IPv4 (via RFC1918 that defines private IP space). Similarly, there is no carrier-grade network address translation (CGNAT). Of course, neither are required in IPv6, but even with an infinite resource, you still have to be diligent about tracking what is on the network. You don’t want to be in a position of having to renumber your network, and furthermore, you should always leave space for customer networks to grow without having to renumber or reassign their allocations.
With this is in mind, the next step is to determine what actually needs to be measured. Metrics that are useful in IPv4 don’t necessarily make sense in IPv6 networks. For instance, measuring the percentage allocated will not give you a clear idea, as the utilization percentage is always going to be close to zero. Host density (HD) ratio is another useful metric; however, it is not an easily communicated representation of usage if you need to explain IP usage to C-level executives or non IP manager technical staff. It’s also worth noting that this equation can also give different results depending on the “p” threshold used in the HD ratio, although RFC 4692 suggests /56 (https://www.ripe.net/publications/docs/ripe-512 – section 5.2.1).
Overall, simple metrics are key. Because IPv6 space is so vast, you don’t really care how much of it is used (as there is always more), you care about what is used. For this, a raw count of the number of IPs used is the best indicator of the pain that would be involved in moving those clients to a different block (effectively renumbering).
In my experience, I’ve found that it’s useful to take a two-pronged approach. The most accurate communication of usage of space is to determine how many blocks are deployed in a particular allocation (which is where HD ratio comes in), as well as raw count for DHCPv6. To do this effectively, you should be looking at a carrier-grade IP address management (IPAM) solution that understands the difference between the two methods and can implement both.
IPv6 is an opportunity to gain infinite address space, but you still need to be strategic about how you plan your IP space. Laying the right groundwork today will put you in good stead for future growth and enable you to expand your network without the pain of renumbering or reallocation.
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