Hello Samir
This is a good point you bring up. When an IP packet is sent, the destination address in the header contains no subnet information. Therefore the receiving router cannot know if it is a directed broadcast. If you don’t know the subnet, you cannot know if it is a network, broadcast, or host address.
However, if the router has some information about the subnet mask of the particular network, then it can indeed determine if it is a directed broadcast. Where can it find this information? In the routing table.
The routing table will have a list of routes to destination networks (directly connected, statically assigned, dynamically learned) where these destination networks have a defined range. The range identifies the subnet mask, and thus the router can know if the destination IP is a directed broadcast or not. Only then will a directed broadcast be dropped.
If a packet is routed based on the default gateway, then there is no way to determine if it is a directed broadcast or not, so it is forwarded normally.
I hope this has been helpful!
Laz