Broadcast addresses

[waseem abdo]

I recently knew that there is 3 types of broadcast addresses which are:

1- Local broadcast address.

2- Subnet broadcast address.

3- Network broadcast address.

Local is the broadcast for the local subnet which we are all already know: 255.255.255.255

Subnet is the broadcast that specific network. For example: 10.1.1.0/24 network , the broadcast is 10.1.1.255 for that network.

Now, Network broadcast stated that network broadcast packet is a packet that reaches all subnets in a network so my question is how is the network broadcast address look like and how is it treated by routers to reaches all subnets in a network even though it is not usual that routers forward broadcast addresses to all of its interfaces (note: subnet broadcast doesnot get forwarded to all routers interfaces only the intended network subnet).

[MHM Cisco World]

2- Subnet broadcast address.

3- Network broadcast address.

Both these are same which is broadcast of 10.0.0.255

Local broadcast is 255.255.255.255

[waseem abdo]

How to send a broadcast address to all subnets in the network?

I am asking this because in the official cert book stated that but did not state how exactly?

[MHM Cisco World]

the router by default drop any broadcast limit or direct 
but the router can config to forward direct broadcast via command (usually use for DHCP farm)

[M02@rt37 · Accepted solution]

Hello @waseem abdo 

From my point of view, the term "network broadcast address" is a bit ambiguous, and in modern networking, traditional broadcast methods have been largely replaced by more efficient unicast and multicast methods. Broadcasts can cause scalability and security concerns, so they are not typically forwarded by routers.

However, if we consider the historical context where broadcasting was more prevalent, the network broadcast address could be conceptualized as an address that reaches all devices within a given IP network. This would mean a broadcast that is not limited to a specific subnet but spans the entire IP network.

If you have a network with multiple subnets and you want to send a broadcast-like message that reaches all devices in all subnets, you might use techniques like directed broadcasts or IP-directed broadcasts. These techniques involve sending a packet to a specific IP address within a network, and routers are configured to forward this packet to all subnets.

As concerned directed broadcast add, suppose you have a network with subnets 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24. The directed broadcast address for this network could be, for example, 192.168.255.255 (assuming the network is 192.168.0.0/16).

When a device sends a broadcast to 192.168.255.255, routers configured for directed broadcast may forward it to all subnets within the 192.168.0.0/16 network.

Routers do not forward standard broadcast packets (like 255.255.255.255) across different subnets due to the potential for broadcast storms and security concerns.

Directed broadcasts, if configured, are an exception and can be used for specific purposes like Wake-on-LAN or time synchronization.

[Joseph W. Doherty]

Network wide broadcast is 255.255.255.255 but routers for quite some time now, by default, don't support such.