Does everyone have their own ASN? How does it work?
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So ICANN assigns ISPs an Autonomous System Number, or ASN. When an ISP is assigned blocks of IP addresses, they get associated to the ASN. Using Border Gateway Protocol, or BGP, ISP’s border routers announce to other border routers that “Hey! If you’re trying to find how to get to this IP address, I’m the one who has it!”, which allows the automatic creation of routing tables. They can then associate the IP address with the ASN who owns it and figure out how to get there by looking up on their routing tables which steps, or hops, need to be taken to get there.
This is simplified, but this is also ELI5 so… Yeah.
EDIT: since I feel like you might ask, there technically isn’t anything that prevents a rouge ASN announcement, as in someone setting up BGP to announce that they own IPs they really don’t. When this happens and becomes a problem, it usually ends up getting researched by network admins who then figure out the true owner(s) and either manually write a BGP entry for it or block the rouge ASN announcing it, depending on the situation. This is not unprecedented. [Here is an article talking about it.](https://www.secplicity.org/2019/01/14/bgp-hijacking-in-a-nutshell/)
If you were to think of the internet as all of the streets in the world, then an AS number would be a city or town and all of the streets within that city.
Each ASN represents an organization and all of the Public IP addresses that are assigned to that organization.
These are typically ISPs but can also be large organizations like Microsoft, AWS, and Rackspace that are large enough to warrant having their own dedicate public IP space.
The AS is used in internet routing (BGP) to determine who has what IP blocks assigned to them, and generating paths and backup paths to get your traffic to those networks.
To use the analogy above, you use it to determine what highways to use to get to that City efficiently.
Not all organizations have AS Numbers because they aren’t large enough to warrant it. Most companies lease their public IPs from an ISP and therefore fall underneath their AS Number.
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