Minimum globally routable address space?

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1

I am writing from a small, multi-homed ISP. We are contemplating
terminating our relationship with one of our providers, and will need to
replace that non-portable space.
We are debating between getting a /20 from ARIN (we have justifiable usage)
or a /20 from our other provider.

On one hand, portability would be nice. On the other, is a /20 globally
routable in practice? I seem to remember hearing that /19 was the smallest
that would not be filtered by anyone.

If anyone knows if there are filters for /20s out there, I would be
interested to know who filters them.

    Thanks,
    Dan Streufert

2

Yes, you'll be fine with a /20 from the blocks that ARIN allocates
from.

When we applied for IP space about a year ago, our /20 came out of 216/8.
ARIN is currently allocating from 64/8, 65/8, and 66/8, so providers
changed their filters to permit that. You can read various provider
filtering policies at http://www.nanog.org/filter.html ; Verio's is
the most restrictive.

Cheers,

Troy