How does routing work? The internet is made up of RENs and ISPs who connect to each other's networks. How does an ISP in Kenya tell an ISP in Japan what end users it has? And how does that ISP send data packets to the customers of the ISP in Japan and get responses back? After all, as on a local ethernet, two-way packet flow is needed for communication between two devices. The ISP in Kenya could buy a direct connection to the ISP in Japan but this does not scale as thousands of ISPs would need thousands of connections and that cost would be astronomical. Instead the ISP in Kenya tells its neighboring ISPs what end users it has and the neighboring ISPs pass this information on to their neighbors and so on. This process repeats until the information reaches the ISP in Japan. This process is called "routing". The mechanisms used are called routing protocols. Routing and routing protocols ensure that the internet can scale and that thousands of ISPs can provide connectivity to each other. This is what we have as the internet we see today. The ISP in Kenya doesn't actually tell its neighboring ISPs the names of the end users as network equipment do not understand names. Instead the ISP will have received an IP block as a member of the original internet registry serving Kenya. The end users will have received address space from this address block as part of the internet service and the ISP announces this address block to its neighboring ISPs. This is called announcing a "route".

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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