Routing between network 1 and subnet of network 2 And their broadcast address

Question

Suppose there's a network A with network 10.0.0.0 with network id=10.0.0.0 and Broadcast address 10.255.255.255 and Mask = 255.0.0.0

and network B with network 197.10.3.0 with network id =197.10.3.0 and Broadcast address 197.10.3.255.

Now If we create 2 subnets in network B with SubMask = 255.255.255.128

Sub1 with range of 197.10.3.0 to 197.10.3.127 AND

Sub2 with range of 197.10.3.128 to 197.10.3.255

Now the Question here is

Ques 1) If a host within network B send a packet with destination address 197.10.3.255 will it broadcast to only Sub2 or whole network

and

197.10.3.0 will point to Sub1 or whole network?

Ques 2) If a host within network A sends a packet with destination address 197.10.3.255 will it broadcast to only Sub2 or whole network

and

197.10.3.0 will point to Sub1 or whole network?

Answer 1

Aside from the problems of your IP math, broadcasts are bounded by routers. The Limited-Broadcast address (255.255.255.255) can never cross a router. A network broadcast address (the "all ones" or highest address in the network) cannot cross a router, by default, but routers may be configured to send directed broadcasts. A directed broadcast is sent to the broadcast address of a different network. Sending something to the broadcast address of your own network will never cross a router.

That means that anything on the 197.10.3.0/25 network to its broadcast address (197.10.3.127) will only ever be seen on that network. If a host on the 197.10.3.0/25 network sends something to the broadcast address of the 197.10.3.128/25 network (197.10.3.255), it will be dropped by the router, unless the router is configured to forward directed broadcasts. The hosts on the local 197.10.3.0/25 network will never see it because it will be sent to the router layer-2 address, not the layer-2 broadcast address.

Remember that layer-3 packets get encapsulated in layer-2 frames, which are used to deliver traffic on the local layer-2 LAN. Ethernet, and other LAN protocols that use MAC addressing, will use the ffff:ffff:ffff broadcast MAC address for broadcasts on the local layer-2 LAN. The host sending to the broadcast address of a different layer-3 network will understand that it is on a different network because of its network mask, and it will use the MAC address of its configured gateway for the frame, not the broadcast MAC address.