Why a broadcast address

amb1s1

Why a Subnet has to have a broadcast address? Because When you send a broadcast, the router never forward the broadcast,

Deadmaster200

Well, think about it for a minute. If a subnetwork doesn't need a broadcast address because the router never forwards it, then why would any network ever need a broadcast ability?

Think and/or research the reasons why broadcasts are beneficial, then you will have your answer.

It doesn't have anything to do with whether it's a subnet or not. Subnets are just ways for us to reorganize logical things that we created to begin with. All the routers see are bits and when a router uses a classless routing protocol that transmits the subnet mask information, the router does not really distinguish or care whether we call them networks, supernets, subnets, or subsubnets, or whatever. A network is a network is a network is a network. The subnet mask just tells the router where to stop reading the bit stream in order to determine the network address of a given IP address.

Paul#4

This is a good question...



What is the purpose of the broadcast address in any given subnet?

A4: The main purpose of a broadcast address in the case of IP is to send out onto the wire a packet that all hosts common to the particular subnets will see and receive. Cisco routers drop broadcasts unless you configure bridging.

The subnet broadcast address is rarely used. We could probably get rid of it and everything would work except for a few protocols(I think DHCP uses subnet broadcast address). IPv4 was designed with multiple networks in mind, but maybe nothing like we see today.


This is a great question....a lot of people never ask why? Sometimes you have to dig to get good answers because most people will respond with vague and misleading answers.

dtlokee

Keep in mind much of the framwork behind how IP works is more than 25 years old and although some ideas (like an IP header checksum) may have appeared to be a good idea they just didn't work out as intended. In the case of the "subnet broadcast" or "directed broadcast" it is rarely used and most devices do not even respond to packets addressed to the broadcast address because of it's potential for abuse in denial of service attacks and such. I have seen it used in cases such as DHCP where there are a few DHCP servers located on a single subnet and the routers (or relay agents) are configured to forward the DHCP discover message to the subet broadcast address for the subnet with the DHCP servers on it, the result? multiple DHCP servers will send a DHCP offer packet, providing for some redundancy.

Netstudent

lets not forget one of the most common broadcast. ARP
IP to MAC resolutions are flying around all the time. Without broadcasts how is connectivity going to occur when the mapping isn't in the arp cache?

Cucumber

amb1s1

There are two types of broadcasts, Layer3 broadcasts and Layer2 broadcasts.

Layer2 broadcasts are always stoped by routers; on ethernet networks, the layer2 broadcasts (stoped by routers) have the address ffff.ffff.ffff.

Now regarding Layer 3 broadcasts, specifically talking about IP, there are two kinds, the local network broadcast 255.255.255.255 and the directed broadcast, like 192.168.1.255.

The local network broadcast (255.255.255.255) will be stoped by routers because this broadcast always maps to layer2 broadcast ffff.ffff.ffff.

Now the directed broadcast CAN be forwarded by routers. Remember the command "ip directed-broadcast". So the broadcast address you calculate when subnetting is usable to generate broadcasts outside your network, but this is discouraged; one usage could be sending a video broadcast from one network to other network, but this is no longer used as Multicasts should be used.