What is an ASN?
Everything you need to know about Autonomous System Numbers — the backbone of internet routing and BGP connectivity.
What is an Autonomous System Number?
An Autonomous System Number (ASN) is a unique identifier assigned to a group of IP networks that are operated by one or more network operators under a single, clearly defined routing policy. ASNs are the fundamental building blocks of the internet's routing infrastructure, enabling networks to communicate their identity and exchange traffic efficiently.
Every network that participates in BGP (Border Gateway Protocol) routing requires an ASN. When your network peers with other networks or announces IP prefixes to the global routing table, your ASN serves as the identifier that distinguishes your network from every other network on the internet.
ASNs are assigned by the five Regional Internet Registries (RIRs): ARIN (North America), RIPE NCC (Europe, Middle East, Central Asia), APNIC (Asia-Pacific), LACNIC (Latin America and Caribbean), and AFRINIC (Africa). Each RIR has its own allocation policies and procedures, but the core requirements are consistent across all regions.
Why Do ASNs Exist?
The internet is composed of tens of thousands of independently operated networks. For traffic to flow between these networks, there must be a standardized way to identify each network and communicate routing decisions. ASNs provide this identification layer, allowing routers to build a map of the internet and determine the best path for each packet.
Without ASNs, it would be impossible to implement peering agreements, establish multi-homed connections for redundancy, or enforce independent routing policies. ASNs are what make the decentralized architecture of the internet possible — every network can make its own routing decisions while remaining globally interconnected.
Unique Identification
Each ASN uniquely identifies a network operator and their routing policy, preventing conflicts and ambiguity in the global routing table.
Routing Policy
ASNs allow network operators to define and enforce their own routing policies, controlling how traffic enters and exits their network.
Multi-homing
Organizations with multiple upstream providers need an ASN to announce their prefixes independently and maintain connectivity if one provider fails.
Peering
Internet exchange points and direct peering agreements rely on ASNs to identify participating networks and establish BGP sessions.
How to Get an ASN
To obtain an ASN, you must apply through a Regional Internet Registry (RIR) or through an LIR (Local Internet Registry) / sponsor that holds a membership with the relevant RIR. The specific process varies by region, but the fundamental requirement is demonstrating a legitimate need for a unique routing policy.
The most common requirement is that your network must be multi-homed — connected to at least two different upstream internet service providers. This ensures that you genuinely need an independent ASN rather than simply using your provider's AS. You'll also need to document your routing policy and provide technical details about your network infrastructure.
The typical timeline for ASN registration is 2–4 weeks, depending on the RIR and the completeness of your application. Costs vary by region: RIPE NCC charges through the Sponsoring LIR, ARIN has a one-time registration fee plus annual maintenance, and APNIC includes ASN resources under its membership tiers. Working with an experienced broker like IPv4Center.com can significantly accelerate the process.
ASN and BGP Routing
ASNs and BGP are inseparable — BGP (Border Gateway Protocol) is the routing protocol that uses ASNs to exchange routing information between networks. When a network announces an IP prefix via BGP, the announcement includes the originating ASN and the complete AS-PATH, which records every ASN the route has traversed.
There are two forms of BGP: eBGP (external BGP) is used between different autonomous systems to exchange routes across network boundaries, while iBGP (internal BGP) is used within a single autonomous system to distribute external routing information to internal routers. Both are essential for large-scale network operations.
The AS-PATH attribute is one of the most important elements in BGP route selection. Routers typically prefer routes with shorter AS-PATHs, as fewer network hops generally mean lower latency and higher reliability. Network operators can also use AS-PATH filtering to accept or reject routes based on which networks they traverse.
eBGP Peering
External BGP sessions connect different autonomous systems, enabling route exchange between networks at internet exchange points or via private interconnects.
iBGP Internal
Internal BGP distributes externally learned routes within an autonomous system, ensuring all routers have consistent routing information.
AS-PATH Attribute
The AS-PATH records the sequence of ASNs a route has traversed, used for loop prevention and path selection in BGP decision-making.
Route Filtering
Network operators use ASN-based filters to control which routes are accepted or advertised, implementing their routing policy and security requirements.
Types of ASN
ASNs come in two sizes: 16-bit and 32-bit. The original 16-bit ASN range (0–65535) was defined in RFC 1930 and quickly became a scarce resource as the internet grew. To address this, 32-bit ASNs, as defined in RFC 6793, were introduced, expanding the total pool to over 4.2 billion unique numbers. Today, most new allocations are 32-bit ASNs, and all modern BGP implementations support them.
ASNs are also classified as public or private. Public ASNs are globally unique and used for routing on the public internet. Private ASNs (64512–65534 for 16-bit, and 4200000000–4294967294 for 32-bit) are reserved for internal use within an organization and must not appear in the global routing table. Private ASNs are commonly used in stub networks that peer with a single upstream provider using BGP.
16-bit ASN
The original format with a range of 0–65535. Most legacy networks use 16-bit ASNs. The pool is nearly exhausted, making them increasingly scarce.
32-bit ASN
Extended format with a range up to approximately 4.2 billion. All new ASN allocations are 32-bit. Represented in "asplain" notation (e.g., AS200001).
Public ASN
Globally unique ASNs assigned by RIRs for use on the public internet. Required for any network that peers with multiple providers or participates in IXPs.
Private ASN
Reserved ranges (64512–65534, 4200000000–4294967294) for internal use. Must be stripped from BGP announcements before reaching the global routing table.
How ASN Numbers Are Assigned
Autonomous System Numbers are allocated by the IANA AS Number registry to the five Regional Internet Registries (RIRs): ARIN (North America), RIPE NCC (Europe/Middle East/Central Asia), APNIC (Asia-Pacific), LACNIC (Latin America/Caribbean), and AFRINIC (Africa). Organizations obtain ASNs from their regional RIR or through a sponsoring Local Internet Registry (LIR). As of 2026, over 115,000 ASNs have been allocated globally.
The original 2-byte ASN range (0–65535) is now mostly exhausted, with very few numbers remaining for new assignments. All new allocations use 4-byte ASNs as defined in RFC 6793, which extends the numbering space up to 4,294,967,295. This expansion ensures that ASN availability will not be a constraint for decades to come, unlike IPv4 address space which reached exhaustion years ago.
Each RIR maintains a free pool of ASNs received from IANA and assigns them sequentially to qualifying applicants. The assignment is permanent — once allocated, an ASN belongs to the holder until it is returned or transferred. RIRs publish allocation data openly, enabling tools like WHOIS and BGP looking glasses to map ASNs to their registered holders.
ASN Use Cases and Requirements
An organization needs its own ASN when it connects to two or more upstream providers (multihoming) and wants to control its routing policy via BGP. Single-homed networks typically use their provider's ASN. To qualify for an ASN from RIPE NCC, an organization must demonstrate a multihoming requirement and have a signed agreement with at least two upstream providers.
Private ASN ranges are reserved for networks that need BGP internally but do not require global uniqueness: 64512–65534 for 2-byte ASNs and 4200000000–4294967294 for 4-byte ASNs, as defined in RFC 6996. Private ASNs are commonly used in large enterprise networks, data center fabrics, and stub networks peering with a single transit provider via BGP.
Beyond multihoming, ASNs are required for participation in Internet Exchange Points (IXPs), operating a Content Delivery Network (CDN), providing transit services to downstream customers, and implementing traffic engineering across multiple upstreams. Cloud providers, hosting companies, and enterprises with global infrastructure typically hold their own ASNs to maintain full control over routing decisions and failover behavior.
ASN Registration at IPv4Center.com
IPv4Center.com provides end-to-end ASN registration services across all five Regional Internet Registries. Whether you need an ASN under RIPE NCC, ARIN, APNIC, LACNIC, or AFRINIC, our experienced team handles the entire process — from initial application and documentation to RIR coordination and final allocation.
For organizations without direct RIR membership, we offer ASN sponsorship services through our LIR accounts. This eliminates the need for your own RIR membership, significantly reducing costs and administrative overhead. Our typical turnaround time is 5–10 business days, and we provide ongoing support for RPKI setup, route object creation, and IRR database registration after your ASN is allocated.
Do I Need My Own ASN?
Whether you need your own ASN depends on your network architecture and how you plan to use IPv4 addresses. Here are four common deployment scenarios:
Scenario 1: Own ASN
Best for ISPs, hosting providers, and enterprises that need full BGP peering control, multi-homing, and independent routing policy. Requires BGP expertise and upstream transit agreements.
Scenario 2: Upstream ASN
Your transit provider announces your IPv4 blocks under their ASN. Simpler setup, no BGP management needed, but less control over routing and no multi-homing capability.
Scenario 3: Cloud BYOIP
Cloud providers (AWS, Azure, GCP) announce your blocks under their ASN. You provide LOA and ROA, they handle BGP. Best for cloud-native deployments.
Scenario 4: Managed Announcement
IPv4Center or another provider manages the BGP announcement on your behalf. You get the benefits of BGP routing without managing the infrastructure.
Frequently Asked Questions
Common questions about Autonomous System Numbers and ASN registration.
You need an ASN if your network connects to multiple upstream providers (multi-homing), if you plan to peer at internet exchange points (IXPs), or if you need to implement an independent routing policy. Single-homed networks that connect through only one ISP generally do not need their own ASN.
ASN costs vary by RIR. Under RIPE NCC, ASNs are obtained through a sponsoring LIR with annual fees typically ranging from €50–€500 depending on the LIR. ARIN charges a one-time registration fee of $500 plus an annual fee of $100. APNIC includes ASN allocations within its membership tiers. IPv4Center.com offers competitive ASN registration packages across all RIRs.
Standard ASN registration takes 2–4 weeks through direct RIR application. With IPv4Center.com's expedited service, most ASN registrations are completed within 5–10 business days. The timeline depends on the RIR, the completeness of your documentation, and whether any additional justification is required.
A private ASN is a number from the reserved range (64512–65534 for 16-bit, 4200000000–4294967294 for 32-bit) intended for internal network use only. Private ASNs are used when you need BGP internally but don't need a globally unique number. They must be removed from route announcements before they reach the public internet.
Yes, ASNs can be transferred between organizations, though the process varies by RIR. RIPE NCC and ARIN both support ASN transfers. The transfer involves updating the registration records to reflect the new holder. IPv4Center.com can facilitate ASN transfers and handle all the administrative requirements with the relevant RIR.
You should register with the RIR that covers the region where your network infrastructure is primarily located: ARIN for North America, RIPE NCC for Europe and the Middle East, APNIC for Asia-Pacific, LACNIC for Latin America, and AFRINIC for Africa. If you operate globally, your primary data center location typically determines the appropriate RIR.