BGP is an exterior gateway routing protocol and it is used to exchange the routes or prefixes between the AS Numbers on the internet.
Before I go ahead let me tell you 3 RFCs to read and also you can read Jeff Doyles TCP/IP Vol-2, This book is one of my favorite and very easy to understand.
RFC4271
RFC4276
RFC4277
Before we learn more about the BGP, The first thing we need to understand is what is an AS Number and How AS numbers plays role in BGP.
What is an ASN(Autonomous System Number)?
BGP is used for exchanging the routing information between the AS numbers.
So basically an ASN is a group or collection of IP networks with having a single technical administrative domain for maintaining the global routing policy on the internet. So AS numbers are globally unique and each and every ASN across the globe is connected with each other directly or via transiting through someone else’s AS and that’s what is called the internet.
Network operators need Autonomous System Numbers to control routing within their networks and to exchange routing information with other Internet Service Providers on the internet.
Who Allocates the AS Numbers?
AS numbers are allocated by the internet registry and if you want an AS number then you can apply for the same but there are lots of terms and conditions and you should be eligible to get the AS number.
IANA is responsible for allocating the resources such as Public IP, AS Numbers, and also for DNS root addressing. But actually, under the IANA, there are a few regional and national registries that allocate these resources. So you have to contact your national internet registry for allocating the IP address or ASN etc. Find the link given below for allocated ASN ranges to the registries.
https://www.iana.org/assignments/as-numbers/as-numbers.xhtml
For Example, IANA allocates address space to APNIC to be redistributed throughout the Asia Pacific region. and here are the details are given below. In India our NIR is IRINN.
IANA allocates address space to RIRs NIRs and also delegates to them the authority to make assignments and allocations. In some cases, RIRs directly assign address space to end-users. But actually, in the last amendment, NIRs allocate and assign address space to their members and customer under the guidance of RIRs and according to various relevant policies and procedures.
In India, IRINN is our national internet registry and they take care of the allocation so any kind of resources you need you can request IRINN directly.
Find the regional internet registry details.
- African Network Information Center (AFRINIC)
- American Registry for Internet Numbers (ARIN)
- Asia-Pacific Network Information Centre (APNIC)
- Latin American and Caribbean Network Information Centre (LACNIC)
- Réseaux IP Européens Network Coordination Centre (RIPE NCC)
Internet registries are governing bodies and they allocate and maintain the database of all allocated resources such as ASN, IP prefixes, and other internet resources as well.
There are a few AS numbers reserved for special use, and We have a private and public ASN range. Private AS is not recognizable on the internet but public ASN are globally unique and can be recognized on the global internet.
Autonomous system numbers are 16-bit which means we have 1- 65535 numbers. Just like private and public IP addresses, we have a range of public and private AS numbers. The Ranges from 1 – 64511 are public ASN and ranges 64512 – 65535 are private autonomous system numbers.
BGP Prerequisite.!
As we know that BGP is used for routing between the AS numbers and running BGP is the only way to be a part of global internet routing.
So the first thing you need for BGP is the public IP address and to get the public IP address, you need to contact your regional internet registry.
The second thing we need for BGP is an AS number and to get the public ASN, you need to contact your regional internet registry ..