No subject

Tue Nov 12 13:18:13 UTC 2002

--On 11 November 2002 18:40 -0800 Harsha Narayan <hnarayan at cs.ucsd.edu> 
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