4.3.1. Obtaining an autonomous system number

The Border Gateway Protocol (BGP) requires that gateways have a special identifier called an autonomous system number (ASN).[39] Most sites do not need to run BGP. Even when a site does run BGP, it usually runs it using the ASN of its ISP or one of the ASNs that have been set aside for private use, which are the numbers from 64512 to 65535. Coordinate your ASN selection with your border gateway peers to avoid any possible conflicts. If you connect to the Internet through a single ISP, you almost certainly do not need an official ASN. If after discussions with your service provider you find that you must obtain an official ASN, obtain the application from the Regional Internet Registry that services your country.

[39]Refer to the section Section 2.3, "Internet Routing Architecture" for a discussion of autonomous systems.

• If you're in the Asia and Pacific region, served by APNIC, you should use the application form at http://ftp.apnic.net/apnic/docs/asn-request and mail the completed form to hostmaster@apnic.net.

• If you're in the Americas, served by ARIN, you should use the application form at http://www.arin.net/templates/asntemplate.txt and mail the completed form to hostmaster@arin.net.

• If you're in Europe, served by RIPE, you should use the application form at ftp://ftp.ripe.net/ripe/docs/ripe-147.txt and mail the completed form to hostmaster@ripe.net.

If you submit an application, you are asked to explain why you need a unique autonomous system number. Unless you are an ISP, probably the only reason to obtain an ASN is that you are a multi-homed site. A multi-homed site is any site that connects to more than one ISP. Reachability information for the site may be advertised by both ISPs, confusing the routing policy. Assigning the site an ASN gives it direct responsibility for setting its own routing policy and advertising its own reachability information. This doesn't prevent the site from advertising bad routes, but it makes the advertisement traceable back to one site and ultimately to one technical contact. (Once you submit an ASN application, you have no one to blame but yourself!)

4.3.2. Registering in a Routing Database

If you obtain an official ASN, you must decide whether you need to register in a routing database. If you got your ASN because you're multi-homed, you should register with a routing database. Section 2.3, "Internet Routing Architecture" explains that routing databases are used to validate routing in the new Internet because there is no longer a central core that can be relied on to determine "best" routes. When you obtain an official ASN, you become part of the structure of co-equal routing domains. You assume responsibility for a small portion of the routing burden and you declare that responsibility by registering in a routing database.

There are several different databases that make up the Internet Routing Registry (IRR). In addition to the Routing Arbiter Database (RADB) mentioned in Chapter 2, "Delivering the Data", RIPE, ANS, Bell Canada, and Cable & Wireless all maintain databases. RIPE serves customers in the RIPE region. ANS, Bell Canada, and Cable & Wireless register only their paying customers. RADB is available to anyone.

To register in the RADB, first register a maintainer object. aintainer objects identify the person who will be responsible for maintaining your database entries. Provide the required information, and pay the $200 fee. You must then register the autonomous system as an AS object. Finally, you create a Route object for each route your system will advertise. See http://www.radb.net for detailed information about registering these database objects.

All of the items discussed so far (addressing, subnetting, and routing) are required to configure the basic physical network on top of which the applications and services run. Now we begin planning the services that make the network useful and usable.