Public vs private
Public ASNs are allocated for internet routing. Private ASNs should stay inside your network and are normally removed before routes are advertised externally.
BGP utility
BGP ASN Converter and AS Number Calculator
Convert an Autonomous System Number between ASPLAIN decimal, ASDOT and hexadecimal formats. Use the result when reading BGP tables, checking legacy documentation or preparing router configuration.
ASPLAINModern decimal format used in most BGP configuration.
ASDOTLegacy X.Y notation for 32-bit AS numbers.
Private ASN checkQuickly identify private 16-bit and 32-bit AS ranges.
Convert AS formats
Enter one AS number and select the format you are starting from.
Results
Decimal (ASPLAIN)-
ASDOT-
Hexadecimal-
Range-
How to use AS numbers in BGP
Common private ranges
Use 64512-65534 for 16-bit private ASNs and 4200000000-4294967294 for 32-bit private ASNs in labs or internal routing.
Operational checks
AS numbers appear in the AS_PATH attribute. They help BGP detect loops, apply routing policy, and influence path selection.
AS number conversion workflow
Autonomous System numbers show up in router configuration, BGP tables, route-server output, peering requests, RPKI records, and ISP documentation. The same ASN may be written in ASPLAIN decimal form, ASDOT notation, or hexadecimal form depending on the vendor, age of the document, or troubleshooting tool. Converting the value before changing a router avoids accidental neighbor configuration errors, especially when a 32-bit AS number is copied from old notes into a modern BGP configuration.
Check the expected format
Most current Cisco, Juniper, Arista, and open-source routing platforms accept ASPLAIN by default, such as 65000 or 4200000100. ASDOT appears in older training material and some legacy outputs. When two teams exchange a peering form, converting both formats helps confirm that the local AS, remote AS, and documented route policy all refer to the same network.
Validate lab and private ranges
For internal labs, use private ASNs such as 64512-65534 or 4200000000-4294967294. These ranges are useful for BGP practice, enterprise edge simulations, MPLS VPN labs, and route reflector testing. They should not be leaked to the public internet unless your upstream provider explicitly rewrites or removes them as part of the external policy.
Compare with routing output
After converting an ASN, compare it with the AS_PATH in BGP output. A repeated AS may indicate prepending, a private AS in an external route may indicate a filtering issue, and an unexpected public AS can point to a wrong neighbor, route leak, or stale documentation. The converter gives you a quick reference while reading those paths.
Related BGP resources
- BGP overview explains where AS numbers fit into inter-domain routing.
- BGP path attributes covers AS_PATH, NEXT_HOP, LOCAL_PREF, MED, and communities.
- BGP routing policies shows how AS paths are filtered and modified.
- IP location and ASN lookup helps identify the organization and ASN behind an IP address.
- Routing practice test checks BGP, OSPF, EIGRP, RIP, metrics, and path selection knowledge.
Understand ASPLAIN and ASDOT Formats
An autonomous system number identifies a routing domain that presents a consistent policy to other networks. Modern ASNs are 32-bit values. ASPLAIN writes the number as one decimal integer, such as 65551. ASDOT writes the same 32-bit value as two 16-bit parts, such as 1.15. The conversion is mathematical; it does not change the autonomous system or create a new assignment.
To convert ASDOT high.low to ASPLAIN, calculate high × 65536 + low. To convert ASPLAIN to ASDOT, divide by 65536: the integer quotient is the high part and the remainder is the low part. Values from 0 through 65535 are commonly displayed as a single number because their high part is zero.
Worked ASN conversion examples
ASDOT 1.15 to ASPLAIN
Calculate (1 × 65536) + 15. The result is 65551, so ASDOT 1.15 and ASPLAIN 65551 identify the same ASN.
ASPLAIN 65551 to ASDOT
Divide 65551 by 65536. The quotient is 1 and the remainder is 15, which produces ASDOT 1.15.
16-bit ASN 65000
The high 16-bit part is zero, so this value is normally written as 65000. It also falls within the private 16-bit range used for internal networks and labs.
Use the Correct ASN in Operations
Before configuring BGP, confirm whether documentation, a vendor portal or a device expects ASPLAIN or ASDOT. Compare the converted value with the neighbor configuration and routing output; a format difference can look like a different ASN even when the underlying 32-bit value is identical. Public ASNs are assigned through regional internet registries, while private ranges are intended for controlled internal or lab use and should not be advertised unchanged to the public internet.
- Record both representations when working with systems that display different formats.
- Validate each ASDOT component is between 0 and 65535.
- Do not infer ownership from the number alone; use an authoritative registry or ASN lookup.
- Check the local and remote AS on both sides of a BGP session before troubleshooting timers or policy.
Continue with the ASN Lookup, BGP overview, and BGP configuration lab.