When you make a modem connection to your ISP and want to connect to, for instance, www.bgpexpert.com, all the routers along the way have to know where to send the packets you're sending to our Web server, and the packets from the server have to find their way back to your computer.
For the first few hops, this isn't much of the problem. For instance, your computer only knows the packets don't have a local destination, so they should be sent over the modem connection. This can continue for a while, but at some point the decision where to send the packet next becomes more complex than just "local: keep it" / "not local: send it to a smarter router". The router making this decision will have to know where to send the packet based on the destination IP address contained in it. Since IP addresses are distributed fairly randomly around the globe, there aren't any shortcuts or calculations that make it possible for the router to decide this for itself.
The only way a router can know where to send a packet, is when another router tells it "send those packets to me, I know how to deliver them". The Border Gateway Protocol (BGP) is a protocol that is used between routers to convey this information. Since the routers that talk BGP to each other aren't owned by the same organization (that would kind of defeat the purpose of creating global reachability) this is often called "inter-domain" routing.
BGP and Interdomain Routing Terms
AS
Autonomous System.
AS Number
Autonomous System Number. Each AS has a unique number that is used to identify it in BGP processing.
Autonomous System
An Autonomous System is a network that has its own routing policy. In most cases, customers belong to their ISP's Autonomous System, but multihomed customers obviously have their own routing policy that is different from either ISP so they must be a separate AS.
BGP
Border Gateway Protocol.
EGP
Exterior Gateway Protocol: a routing protocol used between organizations/networks. BGP is an EGP, but there is also an older EGP called EGP.
Gateway
Older term for router. Sometimes the word "gateway" is used to describe a system that connects two dissimilar networks or protocols.
IGP
Interior Gateway Protocol: a routing protocol used within an organization/network. Examples are RIP, OSPF, IS-IS and EIGRP.
Multihoming
The practice of connecting to two or more ISPs. Most multihomed networks run BGP so the rest of the Internet knows where to send packets for the multihomed network even if one of the connections fails.
Router
1. Any system that will receive packets over one network connection and then forward them to another by looking at the network address inside the packet.
2. A special-purpose system (like a computer, but usually without a screen, keyboard and harddisks) that forwards packets.
Routing Policy
A policy that defines how a network is connected to other networks and how packets are allowed to flow.
