so as we've seen so far BGP handles the relationship between autonomous systems an each autonomous system as we've learned is represented by an autonomous system number each organization that's multihoming needs its own unique autonomous system number the IP address space used by the organization when the multihoming comes from either their upstream service provider or from one of the regional internet registries now if we look at the table here we have a range of the autonomous system numbers in use on the public internet the entire 32-bit range is defined in RFC 67 93 and that ranges from zero to just over four billion the whole 32-bit range the original 16-bit AAS number range ran from 0 to 65535 but in 2008 this range was extended to the full 32 bits and we look in this range several of the s numbers or s number sub-ranges have been reserved 0 and 65535 are reserved 1 to 64 thousand for 95 are used for the public internet and we'll find those in use on the public internet today and then after that one we have few small ranges which have been set aside for various uses RFC 53.89 to small documentation ranges one range of 16s numbers for documentation falls in the old 16-bit range and another range of 16 AAS numbers falls in the new 32-bit range we also have the private s numbers going from 6 4 5 1 2 up to 6 5 5 3 4 and we have a s 2 3 4 5 6 which is that transition AS which was reserved for use when organizations are connecting with networks that cannot support 32-bit a s numbers configured on their routers finally the public internet range 650 55 - up to four one nine nine nine nine nine nine nine nine is reserved for the public internet and the regional internet registries are starting to distribute 32-bit AAS numbers from that range and most recently we have some more a s numbers have been reserved those from 4.2 billion up to the entire end of the 32-bit range so they're just under 300 million a s numbers now reserved for private use on networks which are connecting to or using IP protocols the 32-bit range representation is specified in RFC 53 and that defines a s plane which is the traditional format as the standard notation the were proposals to separate the 32-bit a s number into two 16-bit integers but those proposals did not find much favour with network operators and so the industry by and large uses a s plane the standard 32-bit number format for representing a s numbers when talking about router and routing configurations s numbers are distributed by the regional internet registries as I mentioned and it can also be got from upstream service providers who a member of one of the registries and this depends really what the address policies in force within each registry region the entire 16-bit ASN pool has been assigned to the registries by the Ayana and around forty-three thousand of those 16-bit s numbers of on the internet now that's not to say that the other 21,000 have vanished they are there they are in use possibly for private networks offer networks that may be connected to the Internet but we don't have a global view of these ASMs each registry has also received a block of 32-bit numbers an out of 20,000 100 assignments around 16,000 of these are visible on the internet let's at the time of October 2017 this number is slowly incrementing as the regional registry is a handing out address space for the complete list assignments you can refer to the Ayana website as shown in the slides IP addresses are also distributed by the regional internet registries and they're also available from upstream service providers who are members of the registries the entire ipv4 address pool has been exhausted now and each of the five registries are operating in their own ipv4 run out mode the run out mode depends upon which registry region the operators existing in ipv6 address space however is extremely plentiful and the policies in force at the moment are that network operators receive at least us last 32 or v6 address space and n sites our end users will receive at least a / 48 of ipv6 address space so where to get the numbering resources as I mentioned they can be hired from upstream service provider if an Africa they can be hired from AfriNIC Asia and the Pacific region they can be hired from AP NIC for North America from Arin from Latin America and the Caribbean it's like NIC and Europe Middle East and Central Asia is the ripe NCC and the graphic shows approximately the region's covered by each of the regional registries as mentioned when we're talking about it is numbers there is a range of privatized numbers both in the original 16-bit air san range and the new extended 32-bit es number range which has been reserved for private use and these privatized numbers have several applications and we'll look at some of these now a nice be with customers who are multihomed onto their backbone will generally distribute private air snubbers to those customers or may just use one private areas number for all of those customers and we'll look at that scenario later on in this series RFC 2270 describes how a network operator would configure their infrastructure to handle customers who are multihoming onto their backbone primary as numbers could also be used by corporate network or enterprise network with several regions or several offices but maybe the internet connections are only in the core this is more scalable than running an interior gateway protocol over a national or multinational network each of the regions will operate within a private a/s whereas the headquarters or the core of the network would operate in the public s number and of course the internet would only see the public s number another application is within a bgp confederation or again the network operators network infrastructure has been divided into several regions each of these regions would use a private es yet the network operator will be represented globally by it's globally assigned public airs number it's very important that private es ends are not visible on the public internet and so they must be removed from all prefixes which are announced on any peering to the public internet and our recommendation is to include configuration to remove in the external BGP template as we'll see in the BGP best practices discussion and the same as with private address space documented in RFC 1918 private areas numbers are intended for internal use only they must not be leaked or used on the public Internet the Cisco IOS example configuration is simply to put remove private es along with the neighbor command when BGP is being configured
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