IPv6 Subnet Format

One issue that was of particular concern for ISS throughout was that flexibility be incorporated into the design in order to minimise the need for extensive or complicated reallocation in the future. As such, in addition to the high-order bits reserved for future use, the subnet allocation methodology follows whereby bits closest to the boundary between groups are allocated last in order to maximise the potential for reallocation.

Production subnets will use their 12 bits subnet allocation in the following way:

<4 Cat> <8 Physical>

The category group makes up the first 4 bits of the subnet identifier and is reserved for allocating subnets based broadly according to the roles they perform. An example of this would be to allocate subnets for office, public or wireless networks which can each then be further divided in the physical group.

The physical group (8 bits) can then be used to add geographical identifiers to the category. These will, for example, define the different colleges, academic departments and physical buildings that make up the university campus. In practice of course, there are likely to be multiple subnets allocated to larger buildings or departments as necessary.

Address Allocation Mechanism (DHCPv6 vs. SLAAC)

The DHCPv6/SLAAC (stateless address autoconfiguration) issue is still one that is a matter of continued debate among the community at large. After consultation with other 6NET members however, it was determined that there is a strong case to justify the ISS position that stateful DHCPv6 is the most suitable candidate for managed IPv6 address allocation in an enterprise-type network. The benefits of a centralised address allocation and service discovery method for network management make DHCPv6 a better choice than SLAAC for this type of network.

As such, the majority of Lancaster’s IPv6 address allocation requirements will be handled via stateful DHCPv6 but there will be limited use of manually configured addresses (as in the special cases, see above) and SLAAC in certain cases and environments that favour it, such as wireless or mobile networks.