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What are directed broadcast addresses used for in practice?

To be clear, an example of a directed broadcast address is 192.168.1.255 for the network 192.168.1.0/24. On the other hand, the limited (or local) broadcast address is 255.255.255.255, independent of the network address.

Does not use or work with directed broadcasts

  • In DHCP, clients (and optionally servers) use the limited broadcast address 255.255.255.255, according to the RFC.

  • Ping accepts a directed broadcast address, as in ping -b 192.168.1.255 but I noticed that at least on my network, consisting of Linux and Android devices, no machine would answer (they also don't respond to pings using the limited broadcast address but they do reply to unicast pings). This behavior of clients is common, according to this answer. Furthermore, it seems that routers don't forward directed broadcasts per default.

Uses directed broadcasts

  • One application of directed broadcasts I found is for Wake-on-LAN.

Are there other common uses for directed broadcasts in practice?

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    "To be clear, an example of a directed broadcast address is 192.168.1.255 for the network 192.168.1.0/24." If you are on that network, that is the network broadcast address. It is only a directed broadcast if you are on a different network and can direct the packet(s) to the broadcast address of a different network. That is, by default, not allowed by routers to which the target network is attached, although you can configure a router to allow directed broadcasts on specific attached networks.
    – Ron Maupin ♦
    Apr 11 at 12:40

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Yes, by default the last-hop router drops directed broadcasts (most previous hops wouldn't know what it is).

A while back I used them for notifiying clients about new emails (New Mail Check for MDaemon), even across VPN links. Use cases largely vary though, and most often directed broadcasts are considered a security risk and a 'dirty' method today. Since limited and directed broadcasting was removed for IPv6, (routed) multicast is the way forward.

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Most modern networks disable directed broadcasts as they can be used as an effective denial of service attack. Broadcasts for specific applications such as Wake on Lan have to be explicitly enabled.

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