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Reserved ASNs

You landed here because the AS number you looked up is reserved by the IETF or by IANA for a specific technical purpose. Reserved ASNs are not delegated to any operator and have no routing data to display — each one exists only as a sentinel or as a slot that IANA has not yet handed out to the RIRs.

AS0

RFC 7607

“No originating AS.” RFC 7607 forbids a BGP speaker from sending or accepting AS0 in an AS_PATH. AS0 is also used in RPKI “disavowal” ROAs (RFC 6483, RFC 9319): an AS0 ROA is a signed statement that the covered prefix should never be routed by anyone. You will see AS0 ROAs published by operators who have unused IP space and want to prevent hijacks.

AS23456

RFC 6793AS_TRANS

AS_TRANS is the compatibility placeholder that a 4-byte-ASN speaker inserts into the AS_PATH when a 2-byte-only peer cannot represent the real 32-bit origin. When you see AS23456 in a BGP table the real origin lives inside the AS4_PATH attribute, not the AS_PATH. It is never a real network.

AS65535 & AS4294967295

RFC 7300

The last ASN in each of the 16-bit and 32-bit number spaces. RFC 7300 permanently reserves them so implementations have a safe “all-ones” sentinel available for well-known communities like NO_EXPORT (65535:65281) and for future protocol uses. Neither is ever allocated.

AS65552 – AS131071

IANA reserved

The 16-bit ASN pool runs out at 65535. IANA began assigning the 32-bit pool at 131072 to leave a clean buffer above the old numbering space, so the block 65552–131071 is simply un-delegated. Nothing in the RIR delegated-stats files points into this range. Any BGP announcement you see with an origin in this range is a router misconfiguration or a BGP community value that was accidentally mis-decoded as an origin ASN.

Frequently asked questions

What does AS0 mean in an RPKI ROA?

An AS0 ROA is a signed statement that the covered prefix should never be routed by anyone. RFC 6483 calls this a 'disavowal' — the holder of the IP space is telling the world to treat any BGP announcement for that prefix as invalid, regardless of origin. Operators publish AS0 ROAs over unused IP allocations to prevent hijacks.

Why does AS23456 (AS_TRANS) appear in my BGP table?

AS23456 is the compatibility placeholder inserted by a 4-byte-ASN speaker when a 2-byte-only peer cannot represent the real 32-bit origin. The real origin lives in the AS4_PATH attribute. If you see AS23456 as a final origin, your looking glass is not decoding AS4_PATH or the peer chain still has a 2-byte-only router.

Is AS65535 ever used?

AS65535 is permanently reserved as the last 16-bit ASN. It is used in well-known BGP communities like NO_EXPORT (65535:65281) and NO_ADVERTISE (65535:65282). It is never assigned to any operator and should never appear as an origin AS in a BGP announcement.

What is the IANA unallocated pool (65552–131071)?

When IANA began assigning 32-bit ASNs it started at 131072 to leave a clean buffer above the old 16-bit numbering space. The block 65552–131071 is simply un-delegated — nothing in any RIR's delegated-stats file points into this range. Any BGP announcement with an origin here is a misconfiguration.

Why ipctl can’t look them up

ipctl.io derives its ASN database from the RIR delegated-stats files and from live BGP tables. Reserved numbers are never delegated and should not appear in BGP, so there is no operator, no country, and no prefix list to show. When a reserved ASN does show up in a BGP feed we treat it as noise and filter it out of the hijack and MOAS detection pipelines so the false positives do not pollute the BGP stats page.

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