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Re: splitting up a /20 (was: Class B Address)
From: Jeff Mcadams <jeffm () iglou com>
Date: Tue, 6 Jun 2000 14:22:13 -0400

Also sprach William Allen Simpson
Matt Cramer wrote:
My company has a /20 out of the traditional Class C space.  We want
to use those addresses rather than the addresses our ISP would give
us.  We have asked the ISP if they can announce a /24 out of that
block, and they have said "sure".  However, I have read here about
announcement filtering.

Will certain providers filter that /24?  We have two reasons for
using our own space.  First, we can get redundancy by connecting with
two ISPs and having them both announce the network (or have one
announce if the other dies).  

Multi-homing redundancy is a good thing, assuming that you have
undertaken multiple service entrances to your facilities, and prevented
your circuits from being "groomed" into the same cable elsewhere.
Several such concerns have been described recently on this list.

Otherwise, your multi-homing makes no sense.  

To the contrary.  It doesn't make "no" sense...though it may make your
multi-homing less effective than you hoped for.

Experience has shown that failures are more likely to occur in your
local facilities than in the provider(s). 

Maybe your experience...certainly not mine.  The vast majority of
downtime for IgLou is a result of upstream providers of some sort (LEC,
IXC, ISP; usually ISP).  Let's put a YMMV on this, shall we?

Second, we can carve up our /20 in /24s and use them for different
Internet POPs for our company (e.g. one in the states, one in Europe, one
in the Pacific Rim, etc.).

A /20 that is split up into /24s should be filtered!

Yet another overbroad statement.

REMEMBER: IP addresses are related to network TOPOLOGY, not your
company administration.  Dividing a set of "related" addresses into
unrelated topology (split by oceans) increases the routing costs of
everyone else.

Right, if their network topology (divided by oceans) connects to the
Internet in multiple places, there will be multiple places announcing
routes.  Is it more efficient for all involved if they carve a /20 into
smaller blocks, which at least have some chance of being aggregated at
some point, or to use totally seperated blocks that have no chance of
ever getting aggregated?
Jeff McAdams                            Email: jeffm () iglou com
Head Network Administrator              Voice: (502) 966-3848
IgLou Internet Services                        (800) 436-4456

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