Broadcast

[o1n]

Hi everyone, my question is about broadcasts.

An IP address is composed of networks and nodes so a 255.255.255.255 is a broadcast to all hosts on the current network. Then a x.x.255.255 is a broadcast to all hosts on network x.x Finally my question is this: 255.255.x.x according to the book I am reading is a broadcast to "all networks" but how is this possible if routers break up broadcast domains??

Thanks!

[Big Evil]

Routers, by default, break up a broadcast domain, which is the set of all devices on a network segment that hear all broadcasts sent on that segment. Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast—unless you’ve got a router. When the router’s interface receives this broadcast, it can respond by basically saying “Thanks, but no thanks,” and discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, it’s important to remember that they break up collision domains as well. Two advantages of using routers in your network are:


•They don’t forward broadcasts by default.
•They can filter the network based on layer 3 (Network layer) information (i.e., IP address).

HTH.

[omd]

I am sure that the idea of being able to communicate a message to all nodes everywhere seemed like a good idea at the time; something like the Emergency Broadcast System in the U.S. that uses the radio and tv airwaves to get the word out about violent weather storms and other civil emergencies.

But, we can only imagine the network congestion (and failures) that would occur if anyone, anywhere, were really able to send data to everyone, everywhere.

So, while 255.255.255.255 is indeed defined to mean a broadcast to all nodes on all networks, it isn't really implemented.

If we could limit who was allowed to use the all-nodes-on-all-nets destination to a few trustworthy systems and administrators (perhaps government owned ?), then we could, maybe, ensure that its use was not being abused. However, I am not sure that a global broadcast could even be made to work on the Internet as we know it. There are mechanisms in place to prevent routing loops as much as possible and to minimize their impact where loops do exist. These work well for unicast destination addresses but would likely fail miserably for the all-nodes-on-all-nets broadcast address.

[Big Evil]

It was 1982, R. Gurwitz and R. Hinden.

http://www.rfc-editor.org/in-notes/ien/ien212.txt

http://tools.ietf.org/html/rfc919

[omd]

Good stuff, Big Evil.

The second link points out that the notion of an all-nodes-on-all-nets destination is not so much about sending info around the globe as it is about what might be used when a host does not not yet know its own network; for example, when booting from a bootp server.

[Big Evil]

Thanks. Yeah that stuff freaks people out when you are able to quote RFC's. :)