North American Network Operators Group

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keeping the routing table in check: step 1

  • From: Edward B. DREGER
  • Date: Wed Feb 15 22:49:42 2006

Hopefully this thread will be quick and less convoluted. Rather than simply alluding to "one prefix per ASN", I'd like to detail an allocation scheme that works toward that.

Find the largest contiguous block. Split in half. Round to appropriate boundary. Assign. Space at the end of the block is reserved for expansion.

Ignoring special subnets for simplicity:

0/x, 128/x,
64/x, 192/x,
32/x, 96/x, 160/x, 224/x,
16/x, 48/x, 80/x, 112/x, 144/x, 176/x, 208/x

assuming all grow at equal rates. 96/x ends up growing quickly? No problem. Skip 112/x for the time being.

In short, allocate IP space logarithmically. Start with /1 alignment, proceed to /2, then /3, and so on. Keep the array as sparse as possible so an assignment can be extended without hitting, say, a stride 4 boundary.

Perhaps RIRs should look at filesystems for some hints. Imagine a filesystem that's 30% full yet has as much fragmentation as IPv4 space. Something is wrong.

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