To distinguish one autonomous system from another, an AS can be assigned a unique number from 1 to 65,535. The Internet Assigned Numbers Authority (IANA) is responsible for assigning these numbers. Just like the public and private IP addresses defined in RFC 1918, there are public and private AS numbers. If you will be connected to the Internet backbone, are running BGP, and want to accept BGP routes from the Internet, you will need a public AS number. However, if you only need to break up your internal network into different systems, you only need to use the private numbers. Routing protocols that understand the concept of an AS are IGRP, EIGRP, OSPF, IS-IS, and BGP. RIP doesn’t understand autonomous systems, while OSPF does; but OSPF doesn’t require you to configure the AS number, whereas other protocols, such as IGRP and EIGRP, do. Cisco’s BSCI exam spends a lot of time discussing autonomous systems and routing between them. The CCNA exam focuses only on the basics of IGPs.