3. IPv6 Addressing Limitations

With IPv6 it helps to think in terms of the number of subnets, not hosts as we currently do with IPv4. This is because the standard IPv6 subnet size of /64 effectively supports an unlimited number of connected hosts. The /64 subnet size comes from a technique known as EUI-64 whereby the MAC address of an interface is transposed into the IPv6 address's lower half, allowing an IPv6 address to be automatically assigned without collision (i.e. no DHCP for address allocation, as in IPv4). EUI-64 uses 64 bits for this transposition, hence the standard subnet/host boundary is at /64.

IPv6 addresses are written in hexadecimal notation, and this is also used for naming the reverse DNS zones. These zone files tend to be split on what is called the nibble boundary (a nibble is four bits, i.e. one hexadecimal character). The fallout from DNS zone names being limited to nibble boundaries is that when the hierarchy of IPv6 within Oxford university is created, there are potential constraints on where the network separation takes place. The allocation cannot be split at any single bit as can be done with IPv4 (/22, /23, /24, etc). Rather, splitting is constrained to the nibble (four bit) boundaries (/44, /48, /52, /56, /60 and /64). This influences what size IPv6 network is assigned to a department or college, and then of course how many departments or colleges can be supported overall.

Another point to note is that IPv6 has no ARP, but instead uses something called Neighbor Discovery. Related to this is that there's no concept of broadcast traffic. The equivalent is instead multicast to a well-defined group, an example of which happens to be the all hosts on this subnet group. The core details are covered in RFC3513. To deploy IPv6 you should have multicast capable switches.

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