Network Programming and Python
Because these routers can sit between many networks, their routing tables can
become very extensive and they need to be updated continuously. A simplified
illustration is shown in the following diagram:
The preceding diagram gives us an idea of the arrangement. Each ISP gateway
connects an ISP network to the regional routers, and each home broadband router
has a home network connected to it. In the real world, this arrangement gets more
complicated as one goes toward the top. ISPs will often have more than one gateway
connecting them to the regional routers, and some of these will also themselves be
acting as regional routers. Regional routers also have more tiers than shown here,
and they have many connections between one another, which are in arrangements
that are much more complicated than this simple hierarchy. A rendering of a section
of the Internet from data gathered in 2005 provides a beautiful illustration of just
how complex this becomes, it can be found at http://en.wikipedia.org/wiki/
Internet_backbone#/media/File:Internet_map_1024.jpg.