| An AS is any set of
routers with a consistent routing policy running within a single
administrative domain. An AS could be a collection of routers
running a single Interior Gateway Protocols (IGP), or it could be a collection of routers
running different protocols all belonging to one organization. In
either case, to the outside world the whole AS is viewed as one
single entity. Each AS has an identifying number, assigned by an
Internet Registry or a service provider, between 1 and 65535. AS
numbers within the range 64512 through 65535 are reserved for
private use. Because of the finite number of available AS numbers,
justification should be presented before an organization is given an
AS number.
Today, the ARIN.NET enforces a policy whereby organizations
connecting to a single provider and sharing the same routing
policies as their providers use an AS number from the private pool.
These private ASs are stripped at the provider level. Thus, to the
outside world, several individual networks are advertised as part of
one service provider's
network. Organizations connecting to
multiple providers can request an AS number from the public pool.
Organizations that qualify for getting an AS number should provide
an AS name, an organization name, and a technical point of contact.
Routing information between ASs is exchanged via an exterior gateway
protocol such as BGP4, as illustrated in the Figure.
By defining Autonomous Systems, the Internet gains
the manageability and scalability necessary for sustained growth.
Each AS can implement its own set of rules, policies, and IGPs
independent of other ASs.
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