What is an ASN or AS?
An Autonomous System Number (AS number or just ASN)
is a special number from 1 to 65,535. The magic value 65,535 should
tip you off that an AS number is stored in a computer as a 16-bit unsigned
integer. An ASN uniquely identifies an autonomous system that has a
unique routing policy, or is multi-homed. This autonomous system number
is required if you are to run BGP.
The ASN must be unique so that IP
address blocks appear to come from a unique location that BGP can
find and route to. BGP uses Autonomous System Paths (AS Paths) to
determine the shortest route to a destination.
Public and Private ASN
There are public ASN and private ASN. RFC 1930 outlines which are private and which are public.
PUBLIC ASN (1 - 65511)
The ASN in the public range are globally unique and may be announced on the global Internet.
ASN are used to uniquely identify networks or systems of networks which
appear to the outside world to be running a single consistent routing
policy. Prefixes are 'seen' to originate from these public ASN by the
exterior gateway routing protocol. This ensures that routes lead back to
a unique source of a given range of IP addresses.
PRIVATE ASN (65512 - 65535)
The private ASN should not be seen on the global Internet
(they shouldn't be announced via your exterior gateway routing
protocol). Private AS numbers are used by ISP's who use BGP
confederations. Private AS numbers are also used to provide an AS number
to customers with multiple connections who have no connections to any
other Internet service provider.
Obtaining an ASN
An AS-number must be obtained (leased) from one of
the Regional Internet Registries (RIR's). Which registry you obtain your
AS number from is based upon where in the world your network resides
physically, or where your organization is headquartered, and where will
be connecting. You must apply to the RIR to obtain an ASN. ASN's are
usually provided for a 'container fee' which is a fancy way of saying
that they will charge you money to open an account and place the AS number
under your account. The registry needs to cover their operating expenses,
and 'container fees' are one way they do that.
You
will need to use an online registration page, or the downloadable form
commonly called the ASN template to request your AS number (ASN).
Here's where to obtain the template:
Requirements
There are a limited number of ASN available, so the RIR's are
very selective about whom they grant an ASN to. You will need to
demonstrate the following:
That you have a connection to more than one ISP (or will in the next 30 days). This is called being 'multi-homed'.
You will be asked to specify the
exterior
routing protocol used to communicate
with your ISP (this is exclusively
BGP today)
You will be asked to provide the AS numbers (
ASN) of all
your ISP's.
You may be asked to provide the
IP
addresses of your ISP's
routers to
which you connect. You will already know this information as
it will be provided when the ISP provides your circuit, but you
can discover it with a traceroute if the connection is already
working.
You and your ISP must already have an identity on file in the RIR's database
You must have blocks of
IP
addresses that need routing. You obtain
IP
addresses from RIR's as well.
You must demonstrate a need to utilize
BGP with
an
organizationally unique route policy OR be
multi-homed.
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