What is an autonomous system?
The Internet functions as a vast interconnection of networks, and at its core are autonomous systems, which are the significant networks composing the Internet. More precisely, an autonomous system (AS) is akin to a major hub or a group of interconnected hubs with a unified routing policy. Each computer or device that links to the Internet is affiliated with an AS.
Picture an AS as a bustling airport. Just as travelers move from one airport to another until they reach their final destination, data packets traverse the Internet, navigating from AS to AS until they arrive at the AS housing their designated Internet Protocol (IP) address. The routers within that AS then direct the packet to the specified IP address.
Every AS manages a distinct set of IP addresses, similar to how an airport handles flights to and from various destinations. The range of IP addresses under the governance of a particular AS is referred to as its “IP address space.”
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Most ASes establish connections with multiple other ASes. If an AS connects with only one other AS and follows the same routing policy, it might be considered a subsidiary network of the initial AS.
Typically, a single prominent organization, such as an Internet service provider (ISP), a large enterprise technology company, a university, or a government agency, operates each AS.
What is an AS routing policy?
The routing policy of an autonomous system (AS) is a compilation of the IP address space it governs, along with a list of other connected ASes. This information is vital for guiding packets to the correct networks, and ASes share this data with the Internet using the Border Gateway Protocol (BGP).
What is IP address space?
A specific set or range of IP addresses is known as “IP address space,” and each autonomous system (AS) oversees a distinct segment of this space. Alternatively, a cluster of IP addresses can be labeled as an IP address “block.”
Consider a scenario where all the books in a library are cataloged in order, and each section of the library is assigned a specific range: Section A manages books numbered 0001 to 5000, and Section B oversees books numbered 5001 to 10000. If Emily is looking for a book with the code 1234, she will find it in Section A. However, if she is searching for a book with the code 6789, it will be in Section B.
This analogy sheds light on the functioning of IP address space. For instance, let’s say XYZ Bookstore operates an AS and controls an IP address range that includes the address 192.0.2.253. When a device sends a packet to 192.0.2.253, the packet eventually reaches the AS controlled by XYZ Bookstore. If that same device is also sending packets to 198.51.100.255, those packets are directed to a different AS (although they may pass through XYZ Bookstore’s AS in transit).
What are IP address prefixes?
Networking engineers communicate the control of IP addresses by various autonomous systems (ASes) through discussions about the IP address “prefixes” associated with each AS. An IP address prefix defines a specific range of IP addresses. The notation for IP address prefixes, such as 192.0.2.0/24, represents IP addresses spanning from 192.0.2.0 to 192.0.2.255, not from 192.0.2.0 to 192.0.2.24, due to the way IP addresses are written.
What is an autonomous system number (ASN)?
Every autonomous system (AS) is allocated an official number known as an autonomous system number (ASN), much like how each business possesses a unique, official number through its business license. However, unlike businesses, external entities often identify ASes solely by their assigned number.
AS numbers, or ASNs, are exclusive 16-bit numbers ranging from 1 to 65534 or 32-bit numbers ranging from 131072 to 4294967294. They are represented in the format: AS(number). For example, it is estimated that there are over 90,000 ASNs in use globally.
ASNs are mandatory only for external communications involving inter-network routers. Internal routers and devices within an AS may not necessarily require knowledge of that AS’s number, as they exclusively communicate with devices within the same AS.
Before an AS is assigned an ASN by the relevant governing bodies, it must satisfy specific criteria. This includes having a distinct routing policy, meeting a certain size threshold, and establishing connections with more than one other AS. The availability of ASNs is limited, and if distributed too liberally, the supply would be depleted, leading to increased complexity in routing.
What is BGP?
Autonomous systems (ASes) communicate their routing policies to other ASes and routers through the Border Gateway Protocol (BGP). BGP serves as the protocol for routing data packets between ASes. Without this routing information, managing the Internet on a large scale would swiftly become impractical, leading to the loss of data packets or prolonged delays in reaching their intended destinations.
Each AS utilizes BGP to declare the IP addresses under its jurisdiction and the other ASes to which it is connected. BGP routers aggregate this information from ASes worldwide, consolidating it into databases known as routing tables. These tables are instrumental in determining the most efficient paths from one AS to another. When packets arrive, BGP routers consult their routing tables to ascertain the next AS to which the packet should be directed.
Given the multitude of ASes globally, BGP routers continuously update their routing tables. As networks go offline, new networks emerge, and ASes alter the size of their IP address space, all such changes are communicated via BGP to ensure that BGP routers can adapt their routing tables accordingly.
Why is BGP routing necessary? Isn’t IP used for routing?
IP, or the Internet Protocol, is utilized for routing by defining the destination for each packet. BGP assumes the responsibility of directing packets along the most efficient route to their intended endpoint. In the absence of BGP, IP packets would meander unpredictably across the Internet, hopping from one autonomous system (AS) to another, analogous to a driver trying to reach their destination by making random guesses about which roads to take.
How do autonomous systems connect with each other?
Autonomous systems (ASes) establish connections and share network traffic (data packets) through a procedure known as peering. ASes engage in peering by connecting at physical locations referred to as Internet Exchange Points (IXPs). An IXP constitutes a sizable local area network (LAN) equipped with numerous routers, switches, and cable connections.
FAQ’s
What exactly is an autonomous system (AS)?
An autonomous system (AS) is a significant network or a collection of interconnected hubs that form the backbone of the Internet. It operates like a major hub or a network of hubs with a unified routing policy, and every device connected to the Internet is associated with an AS.
What is an autonomous system number (ASN)?
An autonomous system number (ASN) is a unique numerical identifier assigned to each AS, similar to a business license number for companies. ASNs are essential for external communications with inter-network routers and are typically represented as AS(number).
Could you explain what BGP is and its role in networking?
BGP, or Border Gateway Protocol, is the protocol used by ASes to communicate their routing policies with other ASes and routers. It plays a crucial role in routing data packets between ASes, ensuring efficient paths and preventing data loss or delays on a large scale.
Why is BGP routing necessary, considering IP is already used for routing?
While IP (Internet Protocol) is used for routing by specifying the destination for each packet, BGP (Border Gateway Protocol) is essential for directing packets along the most efficient route to their intended destination. Without BGP, IP packets would navigate unpredictably across the Internet, similar to a driver making random guesses about which roads to take.
Conclusion
autonomous systems (ASes) serve as vital networks in the intricate fabric of the Internet. With unique identifiers known as Autonomous System Numbers (ASNs) and efficient routing policies, ASes, operated by diverse entities, ensure the smooth flow of data packets. The Border Gateway Protocol (BGP) facilitates seamless communication among ASes, contributing to the dynamic adaptability of the Internet. ASes, represented by ASNs, play a crucial role in maintaining connectivity and collectively shaping the robust functionality of the global network.
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