Different Broadcast s

[packetkiller]

I learned from one of my Cisco courses that there are two types of broadcasts, one is directed broadcast and one is limited broadcast.
examples:
directed broadcast: 172.16.0.255
limited broadcast: 255.255.255.255

since, by default, router does not forward any broadcast frames, what is the purpose of using these two types of broadcasts in a local network? when should a pc use directed broadcast, when use limited?

if a pc sends an arp. it is 172.16.0.255 or 255.255.255.255?

any input will be appreciated

 

[CiscoGuy33]

packetkiller,

YES, routers do not forward broadcasts by default, but in some cases they can.

A directed broadcast such as 172.16.0.255 is "directed" to ALL nodes on the 172.16.16.0 network whereas the broadcast 255.255.255.255 is "technically being sent to EVERY node in the know world, however since routers do not forward broadcasts by default a broadcast of 255.255.255.255 will be "read" by EVERYTHING within the broadcast domain.

You said - when should a pc use directed broadcast, when use limited?, this really is NOT a PC thing, you don't sit down at your PC and determine that you are going to send a broadcast - it is a programming thing - a programmer determines these kinds of things based on his/her understanding of broadcasts. It is then designed into the software or the OS or the IOS.

An example is a Cisco router will broadcast 255.255.255.255 looking for a TFTP server and certain config files in some siturations, since the programmer who set this up in the IOS has no idea what network the router will be attached to he uses 255.255.255.255 to send to the "known world" or at least as far as it will reach before being stopped by another router, hoping it will find the TFTP and the config file.

It is really more of a programmers thing then a user thing

For practical purposes and testing - just ubderstand the difference between the 2.

Hope this helps, I am sure others will have more to add!


E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[lerdalt]

DHCP is another example of a broadcast. PC will broadcast for the DHCP server when it comes up and starts looking for an ip address. Without some special configuration, the dhcp server and the client would need to be in the same subnet. You can use helper addresses to then convert the broadcast into a unicast request.