ASN Format Converter
Convert an Autonomous System Number between asplain (4200000000) and asdot (64086.59904) in both directions, and see whether it is 16-bit or 32-bit, private, reserved or AS_TRANS.
Input
1–10 digits, optionally prefixed with AS. For example 4200000000 or AS65001.
Two decimal halves separated by a dot. For example 64086.59904.
Quick facts
- Asdot split point
- 65536 (2^16)
- Largest ASN
- 4294967295 (0xffffffff)
- AS_TRANS
- 23456 (0.23456 in asdot)
- Private 16-bit
- 64512 – 65534
- Private 32-bit
- 4200000000 – 4294967294
From asplain
- Asplain
- 4200000000
- Asdot
- 64086.59904
- High 16 bits
- 64086
- Low 16 bits
- 59904
- Hexadecimal
- 0xfa56ea00
- Width
- 32-bit (needs 4-byte ASNs)
asdot high half
asdot low half, unpadded
Classification
- Private use
- Yes — RFC 6996
- Reserved
- No
- AS_TRANS
- No
- Special range
- Private use (32-bit block) (RFC 6996 §5)
Private use (32-bit block) — RFC 6996 §5. Fine internally, never announced to the internet.
How to write it
- IOS / IOS-XE
- router bgp 4200000000
- Junos
- set routing-options autonomous-system 4200000000
- Asdot form
- 64086.59904
A 32-bit ASN is configured in asplain form; IOS only prints the asdot form when a legacy 16-bit peer forces it.
Junos also accepts the asdot form in place of the asplain number.
What some older configurations and BGP community strings still use.
From asdot
- Asplain
- 4200000000
- Asdot
- 64086.59904
- High 16 bits
- 64086
- Low 16 bits
- 59904
- Hexadecimal
- 0xfa56ea00
- Width
- 32-bit (needs 4-byte ASNs)
asdot high half
asdot low half, unpadded
Classification
- Private use
- Yes — RFC 6996
- Reserved
- No
- AS_TRANS
- No
- Special range
- Private use (32-bit block) (RFC 6996 §5)
Private use (32-bit block) — RFC 6996 §5. Fine internally, never announced to the internet.
How to write it
- IOS / IOS-XE
- router bgp 4200000000
- Junos
- set routing-options autonomous-system 4200000000
- Asdot form
- 64086.59904
A 32-bit ASN is configured in asplain form; IOS only prints the asdot form when a legacy 16-bit peer forces it.
Junos also accepts the asdot form in place of the asplain number.
What some older configurations and BGP community strings still use.
The common misconception
A lot of pages state that 4200000000 is 64090.0. That is wrong twice over: the correct asdot form is 64086.59904, and 64090.0 is actually 4200202240 — a different ASN entirely.
64086 × 65536 + 59904 = 4200000000. The high half is the top 16 bits, so 4200000000 ÷ 65536 is 64086 with a remainder of 59904.
Frequently asked
- What is asdot, and why does 4200000000 become 64086.59904?
- Asdot (RFC 5396) writes a 32-bit ASN as high16.low16 in decimal. 4200000000 divided by 65536 is 64086 remainder 59904, so it is 64086.59904. The claim that it is 64090.0 circulates widely online and is simply wrong — 64090.0 is 4200202240, a different ASN.
- Which notation should I configure on a router?
- Asplain is accepted everywhere modern and is what looking glasses print. Asdot still appears on older Cisco and Juniper configurations and in some BGP community formats, so both are offered here.
- What is AS_TRANS (23456)?
- RFC 6793 reserves AS 23456 as the placeholder a 32-bit-capable router substitutes when it must send a 32-bit ASN to a neighbour that only understands 16-bit ASNs. It is not an ASN you can be assigned.
- Which ASNs are private?
- RFC 6996 reserves 64512-65534 for 16-bit private use and 4200000000-4294967294 for 32-bit private use. They work fine inside a network but must never be announced to the internet.
- Is 65535 a valid ASN?
- No. RFC 7300 reserves 65535 (the last 16-bit value) and 4294967295 (the last 32-bit value). AS 0 is reserved by RFC 7607 as well.
Next
- BGP AS-Path Regex TesterTest a BGP AS-path regular expression against a list of real AS paths, one answer per line. Expands the Cisco underscore metacharacter, explains each token in plain language and ships a preset library.
- MAC OUI Vendor LookupPaste a MAC or a bare OUI and get the registered vendor, the assignment type (MA-L, MA-M or MA-S) and whether the address is globally unique or locally administered. Curated offline subset of the IEEE registry.