In order for an organisation to connect to the rest of the internet directly; it must obtain a public Autonomous System Number, or AS Number.
AS Numbers originally were up to 16-bits in length, but were expanded through RFC 4893 to allow up to 32-bits in length due to AS Number exhaustion. This increased the number of available AS numbers from 65,535 to 4,294,967,295.
The Internet Assigned Numbers Authority (IANA) are responsible for assigning public ASNs and ensuring that they are unique from other organisations. IANA have a few requirements before assigning a AS number to an organisation:
- Proof of a public network range allocated to that organisation
- Proof that the public network range is connected through multiple network providers
- There is a need for a unique routing policy that the existing provider can not provide
If an organisation wishes to use BGP privately, it has two ranges it can pick from freely without any need for it to be assigned from an authority:
- 64,512 – 65,535
- 4,200,000,000 – 4,294,977,294
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