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--On 11 November 2002 18:40 -0800 Harsha Narayan <firstname.lastname@example.org> wrote:
How do ISPs manage the allocations they get from the RIRs? More specifically, do they make the assignments from this sequentially or not? Are multihoming assignments to customers amidst non-multihoming assignments? I ask this because /23s and /24s seem to be scattered over a wide area - they are not adjacent to each other.
Some ISPs use allocation strategies (within the block from the RIR) to maximize the likelihood of a future request from the same customer being capable of adjacent assignment in such a manner as to produce aggregatable blocks, to reduce routing entries. The simplest dumb strategy if all requests were of equal size would (effectively) be to reverse the binary bits (for instance when allocating /24s out of a /16 allocate 0.0, 128.0, 64.0, 192.0, 32.0, 160.0, 96.0, 224.0 and so on). Others use more informal strategies (e.g.'well you may well want 2 x /24 but you are only entitled to one x /24 on the basis of the current network plan. We'll give you one now use the adjacent /24 last but if we have to use it in order to get another block from the RIR then tough'). Generally there's only one block (or at most 2) active at a time in most ISPs as the RIR won't issue another until utilization in existing ones is good. However, there is of course reuse of space when customers leave which also distributes address space. Alex Bligh