No subject

Harsha Narayan hnarayan at
Tue Nov 12 22:15:29 UTC 2002

  This assignment algorithm is similar to the allocation algorithm used by
the RIRs themselves to make allocations to ISPs (they call it
"binary-bhop"). Are ISPs as aggregation-conscious as RIRs?

  Do ISPs use a commercial package to make assignments or do they write
their own program or do they do it manually?

  I find that the allocations made by the RIRs are close to each other
i.e. they are scattered over a lesser area. However, the assignments made
by the LIRs are scattered over a wide area - it seems more random.


On Tue, 12 Nov 2002, Alex Bligh wrote:

> --On 11 November 2002 18:40 -0800 Harsha Narayan <hnarayan at>
> 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

More information about the NANOG mailing list