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First of all, I am not sure if I can ask this question here, and, if not, I appreciate that you indicate me the best website to ask this question.

I have been reading about ASs (Autonomous Systems), ISPs (Internet Services Providers) and RIRs (Regional Internet Registries).

For example, I understood that there's a big entity at the top of the RIRs that actually is more powerful than RIRs, we are talking about IANA. This IANA is for example responsible for allocating IP addresses or to give ranges of addresses to RIRs (as far as I have understood).

ISPs are for example the telephone companies that allow you to connect to the internet through their service. They usually give you a modem (and a router), and, when you want to access some website (for example), the request passes through their servers.

ASs, as far as I have understood, can be many ISPs put together or just one. An AS exists because the entities that take part of that AS share the same policies, for example the same routing protocols (e.g., OSPF).

I am not sure if the description I gave are correct, but I am pretty sure they are quite incomplete. Since I have not found nothing really satisfying that describes the difference between autonomous systems, ISPs and RIRs, I decided to ask here a question with the hope of receiving an exhaustive description of what they are and what's the difference between them.

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    You've got it right for the most part. While it's possible for multiple ISPs to be in one AS, I'm pretty sure that is a rare exception. Different companies seldom have identical routing policies -- it's not just what protocols, but deciding which traffic you carry on your network, and who do you hand it off to. This often involves money, so to different companies will naturally have different point of view. – Ron Trunk Jan 14 '16 at 23:28
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it. – Ron Maupin Aug 13 '17 at 0:37
  • Can someone elaborate on the relation between Autonomous Systems and Local Internet Registries? based on the respective description, they pretty much seem to play similar roles. Is there a 1-to-1 mapping between the two? Thanks – Luca Passani Feb 6 '18 at 13:33
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You are pretty accurate.

IANA is the Internet Assigned Numbers Authority. Its function is to make sure that numbers that need to be unique are actually unique. That includes protocol values, IPv4 addresses, IPv6 addresses and AS numbers.

For IPv4, IPv6 and ASNs we have a distributed system: IANA doesn't assign those numbers directly to the users. They allocate big blocks of numbers to the five RIRs, the Regional Internet Registries.

There are five RIRs: ARIN, RIPE NCC, APNIC, LACNIC and AFRINIC. Each RIR has its own region, and in each region the internet community (so not just paying members, everyone can participate) decides on the local policies for further allocation and assignment of those numbers. Giving each internet user their own address would be a bit inefficient, both organizationally and technically, so most addresses are allocated to the members of the RIR. They then further assign numbers to end users etc.

Such members are called LIRs: Local Internet Registries. Often a LIR is an ISP who performs registry services for its own customers, or a large enterprise that is the registry for its departments and/or subsidiaries.

When someone has been allocated or assigned IP address they usually want to route them on the internet. This is done using the BGP protocol. Each network that has its own policies (including transit connections, peerings, local routing policy etc) is an Autonomous System or AS, and needs its own Autonomous System Number or ASN.

When you are a small network and you only connect to one upstream provider you don't have your own routing policy. You follow the policy of your provider. In that case you can let your IP addresses be announced from the ASN of the provider. That is how multiple organisations can appear as one autonomous system: one organisations controlling it and their customers using it. When you have multiple upstreams and/or peerings you need to get your own ASN.

The other way around also happens: one organisation that runs multiple distinct networks. In that case one organisation will have multiple autonomous systems, each with its own ASN.

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