Why are network and subnet broadcasts used?

[cosimodagostino]

Hi at all. I know what they are but I don't know for what purpose to use them if they are a safety hazard. The only thing I have found on the net and that I have not found in any official Cisco manual is that they are used for functions such as "wake-on-lan". What other uses can they be used for?

[balaji.bandi]

there is good video it explains better than words :

https://www.youtube.com/watch?v=bsnMxCSvwIU

good document to start with :

https://www.firewall.cx/networking-topics/general-networking/109-network-broadcast.html

[cosimodagostino]

Hi thanks for your reply; however I understood how it works but I did not understand what are the uses that are made in a network of a subnet and broadcast network? In the video and in the documentation that you have attached me, the uses are not specified; as mentioned above the only use I have seen that is made of a subnet or network broadcast is wake on lan. Will there be other uses or not?

[Joseph W. Doherty]

Realize, often design choices were (and often still are) based on "we might have a need for that".  Further, many early network design choices did take into account using features for abuse of the network (or as you note a "a safety hazard").  Also keep in mind, earlier networks were "slow" and/or had "little" bandwidth (at least compared to today's networks).  I.e. Being able to send just one packet to "notify" a subnet or a whole network, of some issue, was very "efficient".

[Richard Burts · Accepted solution]

The original post asks about uses for the network and subnet broadcast addresses. It mentions wake on lan as a use for this. In addition I would suggest these uses:

- some routing protocols (RIP for example) want to send routing protocol updates to every device in the network/subnet. The broadcast address allows them to do this.

- some monitoring software might want to check on what devices are present in the network/subnet and can achieve this by sending a packet to the broadcast address.

- if a device is configured to obtain an IP address using DHCP, when it boots up it must locate a DHCP server. It does not know the correct address for the DHCP server so it sends a request using the broadcast address as the destination. If the DHCP server is not located in the local subnet then the request must be forwarded to the remote network/subnet, frequently using the broadcast address of the remote network/subnet.

[cosimodagostino]

Thanks for the detailed answer of course there will also be other uses but at the moment I don't care it was to actually understand what was the use of keeping these services active on the router in which cases.Then of course experience will make other uses of it. One thing is certain, however, so far I have not found any book that has talked about what are the possible uses of these broadcasts it seems as if they do not want to reveal secrets. Thanks again

[Richard Burts]

I am glad that my explanation was helpful. You are right that experience would identify other uses for broadcast address.  Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.