An autonomous system or AS for short is
a collection of routers which share the
same routing policy. What this means is
that all the routers have the same view
of the Internet and agree about the best
way to reach any particular destination.
The routers in an autonomous system are
normally owned and managed by the same
group of people and typically each
organization will have one
autonomous system. There are some cases
where an organization might have more
than one AS but that's quite unusual. Each
autonomous system is identified by a
unique autonomous system number or ASN.
ASNs were originally 16-bit numbers but
they have now become 32-bit in addition
to public ASNs there are ranges
reserved for examples in documentation
and ranges reserved for private use.
These can be used when two ASes want to
exchange information with BGP but that
information doesn't need to propagate to
the rest of the Internet. These days AS
numbers are shown as a single decimal
number which is called the AS plain
format. When 32-bit ASNs were first
introduced they were sometimes shown as
two 16-bit numbers separated by a dot
called the AS dot format but that's
rarely seen now. It's also rare to see
the special ASN 23456
which was a transition measure for
software which didn't understand 32-bit ASNs.
So if you want to connect your
network to the internet with BGP you'll
need your own AS number.
Autonomous system numbers are distributed by the
regional internet registries. They're
also available from upstream service
providers who are members
of one of the registries.
The entire 16-bit ASN pool has been assigned to the registries already.
Around 42,000 of these 16-bit AS numbers are visible
on the global Internet today
at the time of this recording.
Each registry has also
received a block of 32-bit AS numbers.
Out of 29,000 assignments made
by the registries around
twenty three thousand eight hundred
of these are visible on the global Internet.
To get a complete list of the AS number
delegations that have been made to the registries
and to end users you can have a look
at the URL displayed on the screen.
Except in a few very limited scenarios
in order to use BGP on the Internet
you must have your own IP address space.
So in fact you'll need three things from
your RIR: a block of IPv4 addresses,
a block of IPv6 addresses
and an autonomous system number.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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