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A brief overview of AS and ASN



An autonomous system (AS)
is a group of IP networks with a clear external routing policy and managed by a network operator. To ensure the interaction of several providers each system must have its own specific identifier, namely an autonomous system number (ASN). 

 

 

Here are 4 types of ASs depending on the connectivity method and operating policy: 

  1. Multihomed: an AS that has connections to more than one other AS, but does not allow transit traffic to pass through to other ASs. This type is usually used for large corporate networks with a number of redundant Internet connections, but which don’t want to pass traffic for others. 
  2. Stub: an AS that is only connected to one other autonomous system. Most ASs with a single Internet connection don't have an assigned ASN, and their network addresses are treated as part of the parent AS.
  3. Multihomed: an AS that has connections to more than one other AS, but does not allow transit traffic to pass through to other ASs. This type is usually used for large corporate networks with a number of redundant Internet connections, but which don’t want to pass traffic for others. 
  4. Internet Exchange Point (IX, IXP): a physical infrastructure that enables Internet service providers (ISPs) and content delivery networks (CDNs) to interconnect and exchange traffic between their ASs. Read more about IXPs in our blog post here.

The Internet connection between networks is performed via the Border Gateway Protocol. This protocol requires a manual configuration, and one of the configuration options includes identification of ASs with which you are establishing a connection. 

An autonomous system number (ASN) is a unique number assigned by the Internet Assigned Numbers Authority (IANA) that is available globally. ASN identifies an AS and allows that system to exchange routing information with other neighboring ASs. 

Companies interested in receiving an ASN have to go through the registration process with RIR (Regional Internet Registry) or LIR (Local Internet Registry). The list of companies with LIR status can be found on the RIPE website.

There are two types of ASNs: 

  1. Public ASNs are required when an AS is exchanging routing information with another AS over the Internet;
  2. Private ASNs are used when an AS is communicating solely via Border Gateway Protocol (BGP) with a single provider. This way, the routing information between the AS and the provider will not be visible on the Internet.

Key Takeaways

  • Autonomous systems can be grouped into four categories (Multihomed, Stub, Transit and IXP) and identified via their unique ASN;
  • Having an ASN is one of the main requirements for a network to connect to an IXP and exchange traffic with other networks through peering.