Natural class B address

Sae writes:

Have question about filtering policy on the natural class B address block.
Trying to use /21 or shorter for various places including international,
from the natural class B block, we have now. Do you see any problem on
announcing as /21 to the Internet?

Yes. Give it back. Renumber into best-fit CIDR blocks from
your provider(s) and/or IRR(s).

Be a good citizen of the global routing tables, or be filtered.


You are going to have a very difficult time getting anything like global
routability like this. /21s out of classical class B blocks are problematic.

Negotiate the return of the /16 to an RIR, but requesting several /20s from
current RIR issued space. You will be a good internet citizen and have
superior routability.

For classical class B space...

/16 - almost certainly routable (nothing is absolute...)
/17-/20 - Decent chance of routability
/21-/24 - Will be filtered by some, but not all.
/25-/32 - filtered by most

- Daniel Golding