Autonomous System Numbers (ASN or AS): BGP Public AS range = 1 – 64,511 (registered with the IANA); Private AS range = 64,512 – 65,535 (typically used for integration with a service provider).
IGP vs. EGP is defined by its relationship to one or more ASNs. Protocols that are designed to work within one AS are Interior Gateway Protocols (IGPs); Protocols designed to work between ASNs are Exterior Gateway Protocols (EGPs).
IGPs: Are typically associated with enterprise internal networks and are not designed to scale beyond a company’s size. They are fast to converge and relatively easy to configure for most operations. Service providers may use IS-IS for interior routing.
EGPs: Exterior Gateway Protocols are designed to control the forwarding of traffic between entities with different ASNs and IGPs in a reliable fashion. BGPv4 is the only EGP in use today. They still rely upon IGPs to resolve routes within an organization’s AS. They scale extremely well and can exchange a huge number of routes.
Stuck-in-Active: Journal of an IT-Network Administrator 