North American Network Operators Group

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Re: IP Prefixes are allocated ..

  • From: Andrew - Supernews
  • Date: Mon Nov 28 02:24:29 2005

>>>>> "Glen" == Glen Kent <[email protected]> writes:

 Glen> to different Autonomous systems.

No. Wrong.

IP addresses are allocated to network providers, or to end-user
networks.  The recipient of a block of IP addresses (by direct
allocation/assignment from ARIN or by a PI assignment from some other
registry) can get their provider to announce the block, in which case
the AS number will change any time they change providers, or indeed
they can get several providers to announce parts of the block. Or they
can choose to get an AS number of their own and announce it
themselves. They might announce part of it themselves and have a
provider announce other parts, and so on.

There is no fixed relationship between addresses and ASNs.

 Glen> Is there a central/distributed database somewhere that can tell
 Glen> me that this particular IP prefix (say x.y.z.w) has been given
 Glen> to foo AS number?

To find out what AS is actually announcing a given IP address, the
place to look is in the routing tables themselves, or information
services which draw their data from routing tables.

One such is

% whois -h
ASN     | IP               | Name
11697   |      | NET-SUPERNEWS - Supernews

Another is the DNS zone:

% host -t txt text "11697" "" "19"

Both of these tell you who _IS_ announcing the space, not who _SHOULD
BE_ announcing it.

Routing registries such as RADB tell you another story; they tell you
who _thinks_ that they _should_ be announcing it or allowing it to be
announced. The quality of such data is poor at best; often it is
nonexistent. Never trust the RADB data to be either correct or

There is currently no fully reliable way for a third party to answer
the question "should AS N be announcing prefix X". The history of
netblock thefts shows that even network providers have a hard time
answering the question "should my customer C be announcing prefix X".

Andrew, Supernews