A complete table of 16-bits and 32-bits ASN
| Number | Bits | Description | Reference |
|---|---|---|---|
| 0 | 16 | Reserved for RPKI unallocated space invalidation[11] | RFC6483, RFC7607 |
| 1 - 23455 | 16 | Public ASNs | |
| 23456 | 16 | Reserved for AS Pool Transition | RFC6793 |
| 23457 - 64495 | 16 | Public ASNs | |
| 64496 - 64511 | 16 | Reserved for use in documentation/sample code | RFC5398 |
| 64512 - 65534 | 16 | Private ASNs | RFC1930, RFC6996 |
| 65535 | 16 | Reserved | RFC7300 |
| 65536 - 65551 | 32 | Reserved for use in documentation and sample code | RFC4893, RFC5398 |
| 65552 - 131071 | 32 | Reserved | |
| 131072 - 4199999999 | 32 | Public 32-bit ASNs | |
| 4200000000 - 4294967294 | 32 | Private ASNs | RFC6996 |
| 4294967295 | 32 | Reserved | RFC7300 |
Understanding BGP 4-byte ASN
BGP Autonomous System Number (ASN) is a 2-byte (16-bits) entity. 2-bytes give 65536 possible numbers, of which, 0, 23456, 65535 and 64512- 65534 (private ASN) are reserved by IANA. Of the remaining, 39000+ ASNs are already used.
RFC 4893 discusses about 4-byte ASN. 4-bytes provide ASNs ranging from 0 to 4294967296. 0-65535 are called mappable-ASN. The 4-byte ASN can be represented in one of three ways-
1. asplain - simple decimal representation of the ASN. For example, ASN 7747 will be represented as 7747, while 123456 will be represented as 123456.
2. asdot+ - breaks the number up in two 16-bit values as low-order and high-order, separated by a dot. All the 2-byte ASNs can be represented in the low-order value. For example, ASN 65535 will be 0.65535, 65536 will be 1.0, 65537 will be 1.1 and so on. The last ASN 4294967296 will be 65535.65535.
3. asdot - it is a mixture of asplain and asdot+. Any ASN in the 2-byte range is represented as asplain and any ASN above the 2-byte range is represented as asdot+. For example, 65535 will be 65535 while 65536 will be 1.0. Cisco uses this form of implementation.
ASN role in BGP
BGP carries ASN in-
- OPEN messages
- AS_PATH attribute
- AGGREGATOR attribute
- COMMUNITY attribute
Terminology
Old BGP Speaker- BGP Speaker running 2-byte ASN
New BGP Speaker- BGP Speaker running 4-byte ASN