If a device wants to broadcast traffic does it send it to the "all ones" IP? (Which is 255.255.255.255)? Or does it send it to "the all ones of the subnet"? (For example if the subnet is 123.123.0.0/16 then broadcast to 123.123.255.255).
If 255.255.255.255 is true then why is 123.123.255.255 left unused?
Note that I'm assuming that a broadcast can't reach outside its subnet. Tell me if this is not always the case.
Also is there a specific term for each one of those "all ones"?
Best Answer
An application can send broadcasts to either the network broadcast address (network all ones) or the limited broadcast address (all ones address).
It used to be the default for routers to allow routing of the network broadcast address (IP directed broadcast), but that proved to be a huge security problem, so routers default to not allow IP directed broadcast. Some routers can be configured to do this.
The limited broadcast, by definition, cannot be routed.
RFC 919, BROADCASTING INTERNET DATAGRAMS and RFC 922, BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS explain broadcasting in more detail.