Broadcast to all subnets

Question

Does usage of network broadcast address will imply broadcasting all subnets? For instance, if my network broadcast address is 192.168.0.255 and I have four subnets, will sending a message utilizing network broadcast cause broadcasting all hosts in all subnets?

Answer 1

No. It will only reach the one subnet that includes this address.

Classful routing is not used today, so a host or router does not infer the parent network simply from knowing the subnets. Therefore, hosts and routers don't know (nor care) that your "whole network" is a /24, and they cannot do any automatic processing that would affect the whole /24.

Additionally, higher-level networks (supernets) don't have broadcast addresses as such. Even if a router is configured to forward the entire /24 network elsewhere, a "gateway" route will not trigger any broadcast processing; if the route points to a gateway, the packet will be unicast to that gateway regardless of the destination address.

Only the lowest-level networks that are directly assigned to a link/an interface (subnets) have broadcast addresses in the first place, and broadcast only comes into play once the packet reaches its near-final destination, i.e the router where the subnet is assigned to an interface. Until that happens, the packet is not a broadcast packet – it is sent as unicast and forwarded as unicast.

For example, if you have a simple network with only one router, and that router has four interfaces configured for your /26 subnets:

• If a host from 192.168.0.192/26 sends a packet to 192.168.0.255, the host recognizes the destination as "local", recognizes that this is a broadcast address of a local subnet, and sends the packet as layer-2 broadcast to the ethernet. Any gateways which receive the packet will not forward it further.

• If a host from 192.168.0.64/26 sends a packet to 192.168.0.255, the host does not recognize the destination as "local" and sends the packet as layer-2 unicast to the gateway.

  The gateway recognizes the destination as "local" for a different interface (the 192.168.0.192/26 interface), recognizes that it's a broadcast address for that subnet, and sends the packet as layer-2 broadcast only through that interface.