Fragmentation of blocks is discouraged. To avoid fragmentation, ISPs are encouraged to have their customers return address space if they change ISPs. Therefore, if the contract between the ISP and the customer terminates, e.g., the customer moves to another service provider, it is recommended that the customer return the network addresses to the ISP and renumber into the new provider's address space. The original ISP should allow sufficient time for the renumbering process to be completed before requiring the address space to be returned. Deployment schedule for the network, including major milestones for each subnet.
Network topology diagrams.
The ISP will then be responsible for the maintenance of IN-ADDR.ARPA domain records of all longer prefixes that have been delegated out of that block. K. Hubbard, M. Kosters, D. Conrad, D. Karrenberg, J. Postel. Internet Engineering Steering Group.