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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?

  • "and network B with network 197.10.3.0 with network id =197.10.3.0 and Broadcast address 10.255.255.255" That is not correct. The network broadcast address must be in the same network. Also, a mask of 255.255.255.192 is a /26 and the ranges you give, 197.10.3.0 to 197.10.3.127 and 197.10.3.128 to 197.10.3.255 are twice as large. Those are /25 networks with a mask of 255.255.255.128. – Ron Maupin Dec 5 '17 at 17:02
  • Thank you for your suggestions. I've made changes you pointed out. – R.s Dec 5 '17 at 17:25
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 20:19
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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.

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