Autonomous Systems and ASNs: The Backbone of Internet Routing
At the heart of the global internet lies a complex web of interconnected networks that communicate through a standardized language. This communication is made possible by Autonomous Systems (AS). An AS is a collection of connected Internet Protocol (IP) routing prefixes managed by one or more network operators on behalf of a single administrative entity or domain. These systems present a common, clearly defined routing policy to the rest of the internet, ensuring data travels efficiently across the globe.
To identify these networks within the Border Gateway Protocol (BGP)—the protocol used to exchange routing information between autonomous systems—each AS is assigned a unique identifier known as an Autonomous System Number (ASN).
Key Facts
- Definition: An AS is a group of IP prefixes under a single administrative control with a unified routing policy.
- Identification: Every AS requires an ASN for BGP routing.
- Governance: ASNs are distributed by the Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs).
- Capacity: The system has transitioned from 16-bit to 32-bit ASNs to accommodate the growing number of networks.
- Growth: The number of allocated ASNs has grown from 5,000 in 1999 to approximately 120,000 by 2025.
The Evolution of ASN Assignment
The management of ASNs follows a hierarchical structure. The Internet Assigned Numbers Authority (IANA) maintains the master registry and assigns blocks of ASNs to Regional Internet Registries (RIRs). These RIRs then distribute the numbers to Local Internet Registries (LIRs) and end-user organizations. As part of the Number Resource Organization (NRO), RIRs also possess the authority to revoke AS numbers.
Historically, ASNs were limited to 16-bit integers, providing a maximum of 65,536 unique assignments. However, as the internet expanded, this limit became insufficient. Since 2007, IANA has assigned 32-bit ASNs. These can be represented in two ways:
- asplain: Written as simple integers ranging from 0 to 4,294,967,295.
- asdot+: Written in an x.y format, where both x and y are 16-bit numbers.
To maintain compatibility between modern "new BGP speakers" (32-bit capable) and older hardware, a special 16-bit ASN, 23456 (AS_TRANS), acts as a placeholder during transitions.
[ไม่มีภาพประกอบ]Classifying Autonomous Systems
Not all autonomous systems serve the same purpose. Depending on their connectivity and operating policies, they generally fall into four categories:
Multihomed AS
A multihomed AS maintains upstream or peering connections to more than one other AS. This redundancy ensures that if one connection fails, the network remains connected to the internet. Unlike transit systems, a multihomed AS does not allow traffic from one AS to pass through it to reach another.
Stub AS
A stub AS is connected to only one other AS. While this might seem inefficient, it is common in specific sectors like finance or transportation where private interconnections exist that are not visible to public route-view servers.
Transit AS
A transit AS acts as an intermediary. Because not every network is directly connected to every other network, a transit AS carries data traffic between two or more ASes. These are typically multihomed to ensure reliability.
Internet Exchange Point (IXP)
An Internet Exchange Point (IXP) is a physical infrastructure where different networks, such as ISPs and Content Delivery Networks (CDNs), exchange traffic. By using an IXP, networks can avoid the high costs of private transit and reduce network latency by shortening the physical path data must travel.
Managing Routing Policies with AS-SETs
To simplify the management of complex routing policies, network operators use AS-SETs. An AS-SET is a collection of ASNs grouped together within an Internet Routing Registry (IRR). Instead of listing every individual ASN in a routing policy, an operator can simply reference an AS-SET.
To prevent conflicts and malicious use, many registries (such as RIPE NCC, APNIC, and ARIN) now enforce hierarchical naming for AS-SETs. This means an administrator might name a set "AS64500:AS-UPSTREAMS" to ensure it is unique and clearly organized within the global registry.
ASN Allocation Summary
| Number Range | Bits | Description |
|---|---|---|
| 0 | 16 | Reserved for RPKI unallocated space invalidation |
| 1–23455 | 16 | Public ASNs |
| 23456 | 16 | Reserved for AS Pool Transition |
| 23457–64495 | 16 | Public ASNs |
| 64496–64511 | 16 | Reserved for documentation and sample code |
| 64512–65534 | 16 | Reserved for private use |
| 65535 | 16 | Reserved |
| 65536–65551 | 32 | Reserved for documentation and sample code |
| 65552–131071 | 32 | Reserved |
| 131072–4199999999 | 32 | Public 32-bit ASNs |
| 4200000000–4294967294 | 32 | Reserved for private use |
| 4294967295 | 32 | Reserved |
Frequently Asked Questions
What is the difference between a 16-bit and a 32-bit ASN?
A 16-bit ASN allows for up to 65,536 unique numbers, whereas a 32-bit ASN allows for over 4 billion unique numbers, providing the capacity needed for the modern internet.
Can I use any ASN for my private network?
No. There are specific ranges reserved for Private Use (such as 64,512 to 65,534 in the 16-bit range) that should not be announced to the global internet.
How are ASNs assigned to an organization?
An organization must apply through their Regional Internet Registry (RIR) or an upstream service provider. Once approved, the RIR assigns a block of ASNs from the pool provided by IANA.
What is the purpose of AS_TRANS (ASN 23456)?
AS_TRANS is a special 16-bit placeholder used to allow 32-bit capable routers to communicate with older routers that only understand 16-bit ASNs.
What is an Internet Routing Registry (IRR)?
An IRR is a database where network operators publish their routing policies and AS-SETs to help other networks understand how to exchange traffic with them.