Routing - Directed vs Local broadcast address

A directed broadcast IPv4 address is an IP address where the host portion of the address is set to all ones in binary.

The local broadcast IPv4 address is 255.255.255.255.

Both of these addresses, when used as the destination address in an IPv4 packet generate broadcast traffic, but they do so somewhat differently.

If a host with an IP address of 192.168.1.55 sends a packet to 192.168.1.255 (the directed broadcast address), it behaves the same as sending the packet to 255.255.255.255 (the local broadcast address). However, the key difference is that 192.168.1.255 allows a host outside the local subnet to send a broadcast to this network segment, whereas 255.255.255.255 restricts the broadcast to the local subnet. Note that the directed broadcast address is always detected as such only by the last router in the path.

Some protocols use the limited broadcast address (255.255.255.255) because they do not yet know the subnet information. Such protocols include DHCP, RARP, and NTP. Other protocols use the directed broadcast address (e.g., 192.168.1.255) because the host sending the broadcast is in a different subnet, and this is the only way to send a broadcast to another subnet. An example of this is the Wake-on-Lan (WoL) protocol.

The choice between limited and directed broadcast addresses depends on the requirements of the specific protocol being used.

Links:

https://forum.networklessons.com/t/ip-directed-broadcast-on-cisco-ios/1327/17?u=lagapidis

https://networklessons.com/ip-routing/ip-directed-broadcast-cisco-ios/