Subnet Broadcast Address?

Ciscopimpenator

Hey TechExam people,

This is the greatest forum! I'm glad I stumbled upon this forum while surfing.

My questions is:

When is the subnet broadcast address used?
I know the IP address 255.255.255.255 is used sometimes but not sure when the subnet broadcast address is used.

Thanks

-Ciscopimpenator

rossonieri#1

hi,

the broadcast really depends on these :
1. power on/off your machine or plug-unplug the cable (after certain amount of time) --> will preduce network broadcast 255.255.255.255 --> your machine registering itself to the network (switch).
2. ip addr + subnet configured in your machine --> if already configure, then your machine do the limited subnet broadcast -- if not then it will do the network broadcast.



cheers.

Ciscopimpenator

But when is the limited subnet broadcast used?
What protocols use the limited subnet broadcast?

For instance, RIP uses 255.255.255.255 to advertise RIP updates, correct?

There has to be a practical use(or maybe not).... otherwise it's a wasted IP address.

Thanks

-Ciscopimpenator

rossonieri#1

hi,

RIP uses 520 for updates.

when you use ip subnet broadcast :
when you try access resources (files,etc) in your LAN such as your WORKGROUP.
your machine will do a subnet broadcast querying for NETBIOS name/ip.

cheers.

Ciscopimpenator

Thanks for the help!

rip udp 520

Cucumber

Hello pimpenator
This is a very good question, the only thing I can add to what have already been said is that
the subnet broadcast address, better known as the "Directed Broadcast" can be used to generate a broadcasts outside a network segment.

A device that is inside a network segment uses the address 255.255.255.255 to generate a broadcast, but that broadcast will be stoped by any router connected to the segment.

Now a directed broadcast, for example, 192.168.1.255 can be generated by a device outside the network segment that has assigned the 192.168.1.0/24 subnet.

Routers **can** receive a directed broadcast and forward them to the destination subnet, however this behaviour is disabled by default, unless using "ip directed-broadcast" command.

Anyway, I can not think of an application that uses this nowadays, besides denial of service attacks, as multicasts are better suited for this kind of stuff.

Ciscopimpenator

Great explanation

And you're right, with a good network design there is little reason to forward directed broadcasts.

I was thinking about this because somebody wanted to connect two subnets via a switch.
We all learn that to connect two seperate subnets you need a router(or layer 3 device) to route the IP.

I kept thinking about this problem and why it wouldn't work, and came up with some interesting reasons.

Thanks

-Ciscopimpenator