Case Study Private AS Numbers

You are familiar with private IP addresses, as specified by RFC 1918. These addresses, in the range 10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255, and 192.168.0.0-192.168.255.255, are designed to help alleviate the depletion of IP addresses. When an internetwork has a need for IP addresses, but the addresses do not need to be known publicly (that is, they do not need to be reachable from the Internet), you can use private addresses rather than public addresses. Because anyone can use any private IP address, tin addresses are not unique and must never be advertised into the public Internet.

Private AS numbers also exist, and like private IP addresses, they are designed to alleviate the depletion of public AS numbers. AS numbers 64512 to 65535 are reserved for private use. If a BGP-speaking subscriber is homed to a single ISP, the subscriber can and is encouraged to use a private AS number.

For example, previous case studies have depicted the autonomous systems connected to router Colorado in Figure 3-26 as "client" autonomous systems of AS 100. AS 2000, connected across a NAP to AS 100, represents the public Internet. AS 100 might be an 1ST and the connected autonomous systems its subscribers, or AS 100 might be the publicly connected part of a large corporate internetwork, and the other autonomous systems its private divisions. Whatever the case, the five "client" autonomous systems in Figure 3-2(> are reachable only across the NAP and through AS 100. The only reason they have individual AS numbers is so that EBGP can be used to connect them to AS 100; AS 100 can advertise their routes to the Internet without including their AS numbers. Figure 3-27 shows the same internetwork as Figure 3-26, but here the "client" autonomous systems use AS numbers out of the private pool.

Remember that like private IP addresses, private AS numbers must not be advertised to the Internet, because they are not unique. Example 3-145 shows that without further configuration, the AS numbers of AS 100's clients are advertised across the NAP to router Austria.

figure 3-27 The Autonomous Systems Attached to Colorado Use Private AS Numbers

f «um pie 3-145 Austria's BGP Table Shows Private AS Numbers in the AS_PATHs of Some of the Routes from AS 100; if Austria Is Part of the Public Internet, Connected to AS 100 Across a NAP, These AS Numbers Must Not Be Included in the ASJPATHs

Austria#show ip bgp

BGP table version is 189, local router ID is 10.2.255.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete