• Collection of network routers running BGP under single administrative domain.
• Every Autonomous system is identified by Autonomous System Number (ASN).
Just like IP addresses, ASNs (Autonomous System Numbers) have to be unique on the Internet.
• 0: reserved.
• 1-64.495: public AS numbers.
• 64.496 – 64.511 – reserved to use in documentation.
• 64.512 – 65.534 – private AS numbers.
• 65.535 – reserved.
Public AS numbers are very limited 1-64.495 and could be exhausted. To avoid this 32-bit AS number has been created which provides 4.294.967.296 AS numbers for use.

IBGP and EBGP
IBGP (interior border gateway protocol)
mechanism to exchange BGP information between multiple BGP border routers in the same AS (Autonomous System) number.
EBGP (Exterior Border Gateway Protocol)
This runs between the router belongs to different AS (Autonomous Numbers).

Basic features of BGP
1. Open standard
2. Exterior gateway protocol
3. designed for inter-AS routing (purpose)
4. Designed for huge inter-network (internet)
5. Classless
Supports: FLSM, VLSM, CIDR auto/manual summary
6. BGP is path vector protocol. It sends route along with path information.
7. Unicast update to manually defined neighbors.
8. Works on Layer-4 TCP port 179
9. Administrative distance
20 (EBGP)
200 (IBGP)