BROCADE IP PRIMER

The Autonomous System (AS)As of 2010, there are five organizations that publicly register IP addresses andAS numbers. Each one covers a geographic region.RegistryAfriNICAPNICARINLACNICRIPEGeographic CoverageAfricaAustralia and most of AsiaNorth AmericaCentral and South AmericaEurope (including Russian Federation)and the Middle EastFor more specific geographic information, consult any of the five registry websites.Each website is “www.” followed by its name, followed by “.net” (e.g.,“www.arin.net,” “www.apnic.net,” “www.lacnic.net,” etc.).It is not essential for you to register a public range of IP addresses or a publicAS number to communicate with the Internet. Very often, ISPs will share a portionof their IP addresses. If you do have a publicly registered range of IPaddresses, you may still advertise them to the Internet under the umbrella ofyour ISP's publicly registered AS number (with your ISP's approval, of course).The larger your administrative entity is, however, you will find that you maywant (or even need) to publicly register a range of IP addresses and a publicAS number.Just like IP addresses, the registries have set aside a special range of AS numbersthat are considered private AS numbers, and are not routed throughoutthe Internet. The private range is 64,512-65,535. Now, where the heck didthey come up with 64,512?! When you're dealing with anything computer ornetworking related, and you wonder why a certain, seemingly arbitrary numberis chosen, convert the number to binary or hexadecimal. You'll find that itmakes much more sense. That rule applies here. In hexadecimal, 64,512 isFFC0. So, from FFC0 to FFFF is private.Why would you use a private AS number? You can use them within your ownentity, certainly (especially if you are using iBGP). More commonly, they areused between an entity and an ISP. Let's say you have a publicly registeredrange of IP addresses. You would like to advertise those IP addresses throughyour ISP to the Internet, but you don't have a publicly registered AS number.Here, you would have to coordinate with your ISP. Your ISP would set up a privateAS, so that you may BGP peer with it. It will receive your routingadvertisements, and pass them along as part of its own publicly registered ASnumber. You would still be using eBGP to talk to your ISP, but your AS number(the private AS number) would not be advertised to the Internet. Instead the IPaddress range would show as being part of your ISP's AS.Brocade IP Primer 275