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Hi I've been learning networks for a while now and I can't seem to find a solution for this doubt online.

I learnt that if you are sending a packet to a class B network's destination 141.49.255.255, it means you are broadcasting the packet to the whole class B network of 141.49.0.0; I also learnt that if you are sending a packet to a class A network's destination address like 10.255.255.255, you are again broadcasting the packet in the network ID 10.0.0.0

My question is, what am I doing when I'm sending a packet to class B 141.255.255.255 ? Am I sending it to a single host or a network?

Joel
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Narayan Dheeraj Kumar
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  • This question is meant for Theoretical Computer Science stack exchange site – Dane Nov 04 '17 at 04:19
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    Network classes are dead, killed in 1993 by RFCs 1517, 1518, and 1519, which defined CIDR (_Classless_ Inter-Domain Routing). Modern networking doesn't use network classes. Please let them rest in peace. – Ron Maupin Nov 04 '17 at 04:41

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Forget classes. Everything is being done in CIDR for the last two decades.

Sending to 141.255.255.255/16 is a subnet broadcast to 141.255.0.0/16. The same address may also be

  • 141.255.255.255/8 - subnet broadcast to 141.0.0.0/8
  • 141.255.255.255/6 - unicast(!) to the node 141.255.255.255

Most often, broadcasts are sent to 255.255.255.255. Routers (usually) don't forward broadcasts, so it's limited to the segment anyway.

However, there are some subtle differences: running e.g. 192.168.1.0/24 and 10.1.0.0/16 inside the very same Ethernet segment, a broadcast to 192.168.1.255/24 is physically sent to all nodes (Ethernet broadcast), even the ones with a 10.1.0.0/16 address. These however will the drop and ignore the packet since it doesn't address them.

In contrast, a broadcast to 255.255.255.255 will be sent to all nodes and processed by all nodes.

Zac67
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