Routing/BGP/CIDR
Transit of packets through the Internet is achieved through routers that support an Exterior Routing Protocol, the most common variation in use being BGP-4 (Border Gateway Protocol; see http://www.ietf.org/rfc/rfc1771.txt/). This protocol is used to communicate routing information between neighbor routers on destinations supported for the routing of data packets.
Routers are able to forward packets to neighboring routers that have advertised their ability to deliver the packet nearer to its final destination. The process of forwarding data to a specific IP is achieved through a Classless Inter-Domain Routing (CIDR) representation. A CIDR network address is shorthand notation that encompasses contiguous ranges of an Internet address or a prefix. For example, the notation 192.168.156.0/24 represents a contiguous address space from 192.168.156.0 to 192.168.156.255, where the number that follows the forward slash represents a bit mask. Routers contain a list of prefixes and neighbors that have advertised their ability to dispatch data nearer to a destination. Routers essentially receive a packet with an explicit Internet address (such as 192.168.156.34), find a match to the prefix, and forward this packet to a nearer neighbor.
The neighbor must know how to forward this packet to another neighbor for it to advertise this prefix. Therefore, packets hop from one router to another as they travel across the Internet and move from a less-specific prefix match to a more-specific one until they find the network that contains the host.
BGP allows for changes in neighbors and routing tables, which can occur often and have a direct affect on the transmission characteristics of a path. This is also one reason that path conditions should always be tested.
—M.L.
