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Figure 2.12. A simple regional (ISP) inter<strong>network</strong>.<br />

In <strong>the</strong> regional inter<strong>network</strong> example, <strong>the</strong>re are two types of interexchange<br />

points.The first is <strong>the</strong> intra<strong>network</strong>/backbone router interexchange point. This is <strong>the</strong><br />

routing demarcation between <strong>the</strong> ISP and <strong>the</strong> intranet. Both <strong>the</strong> ISP and <strong>the</strong><br />

intranet run <strong>the</strong>ir own IGP processes on <strong>the</strong> interexchange gateway router. In <strong>the</strong><br />

case of <strong>the</strong> ISP, <strong>the</strong>y need <strong>to</strong> have an entry in <strong>the</strong>ir routing table for all <strong>the</strong> IP<br />

address spaces that are used by <strong>the</strong> intranet.<br />

As <strong>the</strong> IANA administering party, <strong>the</strong>y are responsible for advertising <strong>the</strong> routes for<br />

all <strong>the</strong> <strong>network</strong>s for which <strong>the</strong>y provide Internet connectivity. Using our earlier LAN<br />

example, <strong>the</strong> routing table entry for <strong>the</strong> ISP would look like this:<br />

Destination Network Mask Gateway<br />

192.160.4.0 255.255.255.0 12.127.247.108<br />

192.160.5.0 255.255.255.0 12.127.247.108

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