Subnet Broadcast Address?

CiscopimpenatorCiscopimpenator Inactive Imported Users Posts: 134
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
-Ciscopimpenator

Comments

  • rossonieri#1rossonieri#1 Member Posts: 799 ■■■□□□□□□□
    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.
    the More I know, that is more and More I dont know.
  • CiscopimpenatorCiscopimpenator Inactive Imported Users Posts: 134
    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
    -Ciscopimpenator
  • rossonieri#1rossonieri#1 Member Posts: 799 ■■■□□□□□□□
    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.
    the More I know, that is more and More I dont know.
  • CiscopimpenatorCiscopimpenator Inactive Imported Users Posts: 134
    Thanks for the help!

    rip udp 520
    -Ciscopimpenator
  • CucumberCucumber Member Posts: 192
    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.
    I hate pandas
  • CiscopimpenatorCiscopimpenator Inactive Imported Users Posts: 134
    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
    -Ciscopimpenator
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