AS (Autonomous System)

An Autonomous System (AS) is a collection of connected Internet Protocol (IP) routing prefixes that are under the control of a single organization or entity. Autonomous Systems are used in the Border Gateway Protocol (BGP), which is used to exchange routing information between different Autonomous Systems on the Internet. In this article, we will explore the concept of Autonomous Systems in more detail, including their structure, functions, and role in the Internet.

Structure of an Autonomous System

An Autonomous System can be thought of as a network that is controlled by a single organization. This organization can be a company, a government agency, a university, or any other entity that operates a network of IP addresses. An Autonomous System is identified by a unique Autonomous System Number (ASN), which is assigned by the Internet Assigned Numbers Authority (IANA). The ASN is a 16-bit integer that ranges from 1 to 65535, with reserved numbers for special purposes.

An Autonomous System can be further divided into sub-autonomous systems, which are also called internal Autonomous Systems. These sub-autonomous systems are used by large organizations to manage their networks more effectively. Each sub-autonomous system has its own ASN, which is assigned by the parent Autonomous System. The use of sub-autonomous systems allows large organizations to distribute their routing policies more efficiently and to manage their networks more effectively.

Functions of an Autonomous System

The main function of an Autonomous System is to control the routing of IP packets within its own network and to exchange routing information with other Autonomous Systems on the Internet. This is accomplished using the Border Gateway Protocol (BGP), which is the standard protocol used for inter-AS routing.

The BGP protocol allows Autonomous Systems to exchange information about the network addresses that they control, as well as the paths that should be used to reach those addresses. Each Autonomous System advertises the network addresses that it controls to its neighbors in the BGP network, along with a list of the paths that can be used to reach those addresses. The BGP protocol allows Autonomous Systems to choose the best path for a particular destination based on a variety of factors, including the length of the path, the number of Autonomous Systems that the path traverses, and the administrative policies of the Autonomous Systems along the path.

The BGP protocol also allows Autonomous Systems to exchange information about the reachability of networks outside of their own Autonomous System. This allows an Autonomous System to learn about the paths that are used to reach networks that are not directly connected to it. This information can be used to make routing decisions that will result in more efficient and reliable network operation.

Role of Autonomous Systems in the Internet

Autonomous Systems play a critical role in the operation of the Internet. They allow organizations to control the routing of IP packets within their own networks and to exchange routing information with other Autonomous Systems on the Internet. This allows packets to be routed between any two points on the Internet, regardless of the number of Autonomous Systems that the packets must traverse.

Autonomous Systems also allow organizations to implement policies that control the flow of traffic through their networks. For example, an organization may choose to implement policies that prioritize certain types of traffic, such as video or voice traffic, over other types of traffic, such as email or file transfers. These policies can be enforced using BGP to control the paths that traffic takes through the Internet.

In addition, Autonomous Systems allow organizations to implement security policies that protect their networks from external threats. For example, an organization may choose to block traffic from certain Autonomous Systems or to implement traffic filtering policies that block traffic from certain IP addresses or ports.

Conclusion

Autonomous Systems are a fundamental component of the Internet infrastructure. They allow organizations to control the routing of IP packets within their own networks and to exchange routing information with other Autonomous Systems on the Internet. This allows packets to be routed efficiently and reliably between any two points on the Internet, regardless of the number of Autonomous Systems that the packets must traverse. The BGP protocol is used to facilitate the exchange of routing information between Autonomous Systems, and allows organizations to implement policies that control the flow of traffic through their networks.

The hierarchical structure of Autonomous Systems, including the use of sub-autonomous systems, allows large organizations to manage their networks more effectively and to distribute their routing policies more efficiently. This helps to ensure that packets are routed through the most efficient paths and that network resources are used in the most optimal manner possible.