Broadcast Addressing Overview

A broadcast is a data packet destined for all hosts on a particular physical network. Network hosts recognize broadcasts by special addresses.

The router supports the following kinds of broadcast types:

• Limited broadcast—A packet is sent to a specific network or series of networks. A limited broadcast address includes the network or subnet fields. In a limited broadcast packet destined for a local network, the network identifier portion and host identifier portion of the destination address is either all ones (255.255.255.255) or all zeros (0.0.0.0).
• Flooded broadcast—A packet is sent to every network.
• Directed broadcast—A packet is sent to a specific destination address where only the host portion of the IP address is either all ones or all zeros (such as 192.20.255.255 or 190.20.0.0).

Several early IP implementations do not use the current broadcast address standard. Instead, they use the old standard, which calls for all zeros instead of all ones to indicate broadcast addresses. Many of these implementations do not recognize a broadcast address of all ones and fail to respond to the broadcast correctly. Others forward broadcasts of all ones, which causes a serious network overload known as a broadcast storm. Implementations that exhibit these problems include systems based on versions of BSD UNIX before version 4.3.

Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges (including intelligent bridges), because they are layer 2 devices, forward broadcasts to all network segments, thus propagating all broadcast storms.

The best solution to the broadcast storm is to use a single broadcast address scheme on a network. Most IP implementations allow the network manager to set the address to be used as the broadcast address. Many implementations of IP, including the one on your router, can accept and interpret all possible forms of broadcast addresses.