Private vs. Public Autonomous System Numbers (ASN): What’s the Difference?

In the world of internet networking, Autonomous System Numbers (ASNs) play a crucial role in routing data efficiently between different networks. ASNs are unique identifiers assigned to Autonomous Systems (AS)—large networks or groups of IP addresses that operate under a single organization, such as ISPs, cloud providers, or large enterprises.

However, not all ASNs are the same. They are divided into public ASNs and private ASNs, each serving different purposes. In this blog post, we’ll explore the differences between private and public ASNs, their use cases, and when to use each one.

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What is an Autonomous System Number (ASN)?

An ASN is a unique 16-bit or 32-bit number assigned to an Autonomous System (AS) by a regional internet registry (RIR) such as ARIN, RIPE NCC, or APNIC. ASNs are used in Border Gateway Protocol (BGP) routing to help different networks communicate efficiently over the internet.

Each AS has its own routing policies and prefix announcements, making ASNs essential for directing traffic across the global internet.

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Public ASNs: What Are They?

A public ASN is an ASN that is globally unique and registered with an RIR. These ASNs are used by networks that need to exchange routing information with other autonomous systems over the public internet.

Key Features of Public ASNs:

Used for external BGP (eBGP) routing between organizations.
Uniquely assigned and registered to prevent conflicts.
Required for ISPs, data centers, and large enterprises that operate their own public routing policies.
Publicly visible in BGP routing tables.

Public ASN Range:

• 16-bit ASNs: 1 – 64,511
• 32-bit ASNs: 131,072 – 4,294,967,295

Who Needs a Public ASN?

• Internet Service Providers (ISPs) that provide connectivity to customers.
• Cloud and hosting providers that route traffic across multiple data centers.
• Large enterprises with multi-homed networks using BGP for redundancy.

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Private ASNs: What Are They?

A private ASN is used for internal routing within an organization and is not publicly visible on the internet. Private ASNs are commonly used for internal BGP (iBGP) routing, private network interconnections, or when connecting to upstream ISPs that handle public routing on behalf of the customer.

Key Features of Private ASNs:

Used for internal routing and private network management.
Not advertised on the public internet (must be stripped before public BGP announcements).
Commonly used for enterprise networks, VPNs, and MPLS configurations.
Can be reused by different organizations since they are not globally unique.

Private ASN Range:

• 16-bit Private ASN Range: 64,512 – 65,534
• 32-bit Private ASN Range: 4,200,000,000 – 4,294,967,294

Who Uses Private ASNs?

• Enterprises managing internal BGP routing.
• Organizations using MPLS or private WAN networks.
• Customers connecting to an ISP via BGP but not needing a public ASN.

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Private vs. Public ASNs: Key Differences

Feature	Public ASN	Private ASN
Uniqueness	Globally unique	Can be reused by multiple organizations
Visibility	Publicly visible in BGP routing tables	Not visible on the public internet
Use Case	ISPs, cloud providers, multi-homed enterprises	Internal BGP, MPLS, private WANs
Routing Type	External BGP (eBGP)	Internal BGP (iBGP)
Registration	Assigned by RIRs (e.g., ARIN, RIPE, APNIC)	No registration required

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When to Use a Private vs. Public ASN?

• Use a Public ASN if:
  You are an ISP, data center, or enterprise needing direct BGP peering with other networks.
  You need to announce IP prefixes over the public internet.
  You operate a multi-homed network with multiple upstream providers.
• Use a Private ASN if:
  You are using BGP for internal routing only (iBGP).
  You are connecting to an ISP that will handle BGP announcements on your behalf.
  You are setting up an MPLS or private network that doesn’t require public visibility.

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Final Thoughts

Understanding the difference between private and public ASNs is crucial for network engineers, ISPs, and enterprises managing BGP routing.

Key Takeaways:

Public ASNs are required for organizations that need to exchange routing information over the public internet.
Private ASNs are used for internal routing and are not publicly visible.
Choosing the right ASN depends on whether your network needs to participate in public BGP peering or private routing.

If you’re setting up BGP for your organization, make sure you choose the right ASN type based on your networking needs and routing policies.

Need help with BGP and ASNs? Drop your questions in the comments!