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

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`. Dec 5, 2017 at 17:02
• Thank you for your suggestions. I've made changes you pointed out.
– R.s
Dec 5, 2017 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. Feb 19, 2018 at 20:19

## 1 Answer

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.