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Basic Internet Architecture R1 IP backbone Network 1 128.80/16 Subnetwork 1 128.80.1/24 Subnetwork 2 128.80.9/24 Figure 4.10 Subnetting allows aggregation within a network single prefix (128.80.0.0/16) to the outside world, and takes care of forwarding packets to whatever subnets have been internally carved from the 128.80.0.0/16 address space. Subnets may themselves be internally subnetted, with increasingly longer prefixes. Taken to an extreme, a subnet may map directly to a single link and have only two members (the IP interfaces at either end of the link). The IPv4 address 255.255.255.255 is a special address meaning broadcast to all hosts on the local subnet . Packets to 255.255.255.255 are never forwarded beyond the IP subnet on which they originate. A more general form, known as the directed broadcast address, is constructed by setting the host part of an IP address to ones. For example, you could transmit a packet to members of subnet 128.80.1.0/24 by using a destination address of 128.80.1.255. Because of the potential for remotely triggered mischief, routers are often set to filter out directed broadcast packets. 4.2.2 Routing Protocols Network topologies change frequently, may be due to human interventions or the usual unpredictable failures that bedevil any large-scale system. Routing protocols must perform a number of tasks such as the following in a timely manner: Dynamically discover a network s topology, and track the topology changes that occur from time to time. Build shortest-path forwarding trees. Handle summarised information about external networks, possibly using different metrics to those used in the local network. The Internet uses distributed routing protocols, which push topology discovery and route calculation processes out into every router. Since the processing load is shared across all routers, sections of the network can continue to adapt locally to changing conditions even if they become isolated from the rest of their network. Figure 4.11 illustrates how every router participates both in forwarding packets (on the basis of previously calculated rules) and in performing distributed routing calculations (updating the forwarding rules as necessary). The detailed art of IP routing is beyond the scope of this book, so we will only briefly summarise a few routing protocols used in the Internet. 4.2.2.1 Shortest-Path Routing When multiple paths exist between a source and a destination, IP networks use shortest- path routing to pick one particular path. The length of a path is typically measured in

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