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    <p>First of all, you should consider broadcast obsolete, specially <code>INADDR_BROADCAST</code> (255.255.255.255). Your question highlights exactly one of the reasons that broadcast is unsuitable. It should die along with IPv4 (hopefully). Note that IPv6 doesn't even have a concept of broadcast (multicast is used, instead).</p> <p><code>INADDR_BROADCAST</code> is limited to the local link. Nowadays, it's only visible use is for DHCP auto-configuration, since at such time, the client will not know yet in what network it is connected to.</p> <p>With a single <code>sendto()</code>, only a single packet is generated, and the outgoing interface is determined by the operating system's routing table (<code>ip route</code> on linux). You can't have a single <code>sendto()</code> generate more than one packet, you would have to iterate over all interfaces, and either use raw sockets or bind the socket to a device using <code>setsockopt(..., SOL_SOCKET, SO_BINDTODEVICE, "ethX")</code> to send each packet bypassing the OS routing table (this requires root privileges). Not a good solution.</p> <p>Instead, since <code>INADDR_BROADCAST</code> is not routed anyway, you can achieve almost the same thing by iterating over each interface, and sending the packet to its broadcast address. For example, assuming that your networks have 255.255.255.0 (/24) masks, the broadcast addresses are <strong>192.168.1.255</strong> and <strong>192.168.2.255</strong>. Call <code>sendto()</code> once for each of these addresses and you will have accomplished your goal.</p> <hr> <p><strong>Edit:</strong> fixed information regarding to <code>INADDR_BROADCAST</code>, and complementing the answer with information about <code>SO_BINDTODEVICE</code>.</p>
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    1. COif INADDR_BROADCAST ever did mean that it was a _very_ long time ago. I'll ask the author of RFC 1 when I see him tonight :) In recent memory it has always mapped to the local network segment's MAC address. _Directed_ broadcasts used to be handled by routers, but are now blocked for security.
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    2. COOk, I checked (RFC 919) and it seems that the first definition of 255.255.255.255 was "immediate neighbors". For some reason I believed that the whole internet was represented by 0.0.0.0/0, resulting in network=0.0.0.0 and brdcast=255.255.255.255. Fixed the answer.
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    3. COok, glad that's fixed. Now, on the comment on IPv6, don't forget that a broadcast is semantically equivalent to a TTL 1 multicast, except that it doesn't use broadcast frames at layer 2.
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