12.5 Summary

Broadcasting is sending a packet to all hosts on a network (usually a locally attached network) and multicasting is sending a packet to a set of hosts on a network. Basic to these two concepts is an understanding of the different types of filtering that occur when a received frame passes up a protocol stack. Each layer can discard a received packet for different reasons.

There are four types of broadcast addresses: limited, net-directed, subnet-directed, and all-subnets-directed. The most common is subnet-directed . The limited broadcast address is normally seen only when a system is bootstrapping.

Problems occur when trying to broadcast through routers, often because the router may not know the subnet mask of the destination network. The results depend on many factors: which type of broadcast address, configuration parameters, and so on.

A class D IP address is called a multicast group address. It is converted to an Ethernet address by placing its lower 23 bits into a fixed Ethernet address. The mapping is not unique, requiring additional filtering by one of the protocol modules.