Broadcast routing

There are two types of IP broadcast.

One is Limited Broadcast, the other is Directed Broadcast.

Limited Broadcast

It is what the destination IP address is 255.255.255.255.In this case, the destination MAC address of the L2 is ffff: ffff: ffff and all will be F.

Direceted Broadcast

This is the IP address leaving the NW address part of the IP address as it is, and host address part as all 1. For example, Directed broadcast of 192.168.1.0/24 is 192.168.1.255.

Well, there is something often mistaken, in devices that are routing configuration of 192.168.1.0/24, do you think the packet destined to 192.168.1.255 can be routed by default?

The correct answer is "possible".For that matter, it is also routed 192.168.1.0. To be precise, if the device "other than" having 192.168.1.0/24 as directly connected route, it can be routed.

Because 192.168.1.0/24 set in routing table does not necessarily indicate that there is ahead in the 192.168.1.0/24 segment. In some cases, there may be two of 192.168.1.0/25 and 192.168.1.128/25 to its destination.

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Assuming that there were two addresses of /25 as above, 192.168.1.127 must be programmed to the directed broadcast address, but it is not known for the routing device such a thing. 192.168.1.128 is similar, this is the NW address, but it is unknown for the device that such a thing 192.168.1.128/25 is directly connected.

In other words, routing entry "192.168.1.0/24 NextHop:XXXX" does not mean that "Beyond the NextHop:XXXX, there is NW address of 192.168.1.0/24" but means the "there are hosts assigned the IP of 192.168.1.0 ~ 192.168.1.255".

If any device is directly connected to 192.168.1.0/24 , then the destination MAC address is set to ffff: ffff: ffff , and sent as directed-broadcast by the device.

ip directed-broadcast

For example, assuming that IP address "192.168.1.1/24" is assigned to FastEthernet 1/1 .

And add the following settings.

(config-if) # ip directed-broadcast

Then, when the directed-broadcast address of 192.168.1.255 has been reached to the device by routing, it will be broadcasted from the interface which above command is set.

This is what often is used in WAKE on LAN.

Security is not the best (which may act as a reflector of DoS attacks), but is a very useful, depending on usage.

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