1

Please validate my understanding for the following points with regard to broadcast address:

1.Broadcast address is relative to a network, so every n/w will have it's own broadcast adress(I can obtain the broadcast address, by replacing all the n/w bits to 1)

2.When the destination IP address is a broadcast address,the router will send out the packet(by replicating it) out of all available interfaces. That means, if a router has interface f0/1 , f0/2 and f0/3 --then 3 packets will be sent out(each being sent out of one of the interfaces)

3.A frame with destination layer 2 broadcast address ( FF:FF:FF:FF:FF:FF) is sent to all the devices in the SAME LAN

  • 1
    Routers bound broadcast domains, so they do not replicate broadcasts to other interfaces. Switches do that for any interfaces in the same broadcast domain (VLAN), but routers do not. Any broadcast reaching a router is stopped at the router. The entire Internet would fail if routers forwarded broadcasts. – Ron Maupin Nov 21 '16 at 16:00
  • See this answer for how to properly calculate a broadcast address for any size network (among other things). – Ron Maupin Nov 21 '16 at 16:03
  • my doubt is clarified sir. I did a small lab set up, with 3 routers linearly connected(R1,R2 and R3) R1 having IP address(1.1.1.1/8) . R2 having IP (1.1.1.2/8 and 2.2.2.1/8). R3 having IP as 2.2.2.2/8. Now from R2 i ping 255.255.255.255 (The result being i receive a unicast reply from 1.1.1.1 and 2.2.2.2) . Now from R2 when i ping 2.255.255.255( i receive a unicast reply from 2.2.2.2 ONLY) – fsociety Nov 21 '16 at 16:40
2

I can obtain the broadcast address, by replacing all the n/w bits to 1)

Actually you set the host bits to 1, not the network bits.

Your other points are OK.

2

1.Broadcast address is relative to a network, so every n/w will have it's own broadcast adress(I can obtain the broadcast address, by replacing all the n/w bits to 1)

Not quite, for any given subnet, the Broadcast Address is found by taking a hosts IP address, and seting any bit positions which hold a '0' in the subnet mask to '1'.

For example, the broadcast address of 192.168.1.1/24 is 192.168.1.255 because the subnet mask defines the host bits as the last octect, and if they are all changed to 1 in binary, the end result is 11111111 or 255.

Since a "Network" may refer to an autonomous system larger than a subnet, the beginning of your question might be better phrased as as a "A Broadcast Address is relative to the subnet on which it originates...".

Source: https://tools.ietf.org/html/rfc919

2.When the destination IP address is a broadcast address,the router will send out the packet(by replicating it) out of all available interfaces. That means, if a router has interface f0/1 , f0/2 and f0/3 --then 3 packets will be sent out(each being sent out of one of the interfaces)

Not exactly, you're describing how an IP packet is forwarded out physical interfaces, but without describing the IP Adresses of those interfaces, there is not enough information to determine how a broadcast packet would be forwarded.

For example, the router may have different subnets available to it on each one of those fast ethernet interfaces, and each subnet has it's own broadcast domain.

3.A frame with destination layer 2 broadcast address ( FF:FF:FF:FF:FF:FF) is sent to all the devices in the SAME LAN

That is correct, and it is worth noting that this is how an ARP request works. If an IP Address needs to be resolved into a MAC address, a broadcast is sent out to all the hosts requesting a reply from the host which has the correct IP address. The host responds in the affirmative with it's MAC address, and in that way the IP is resolved to the MAC.

  • with regard to point 2: assume a router has 3 interfaces(f0/1,f0/2 and f0/3) .Each of them has the ip address as 1.1.1.1/8 , 2.2.2.2/8 and 3.3.3.3/8 respectively . Q1: if the destination IP address is 255.255.255.255----->which all interfaces will the packet sent out from? Q2:if the destination IP address is 3.255.255.255 ----->which all interfaces be the packet sent out from? – fsociety Nov 21 '16 at 16:00
  • Q1 most routers won't send that out at all Q2 It will only be sent on the subnet that the address refers to, in your case f0/3 – marctxk Nov 21 '16 at 17:38
  • did a small lab on GNS3 and the result is as: I did a small lab set up, with 3 routers linearly connected(R1,R2 and R3) R1 having IP address(1.1.1.1/8) . R2 having IP (1.1.1.2/8 and 2.2.2.1/8). R3 having IP as 2.2.2.2/8. Now from R2 i ping 255.255.255.255 (The result being i receive a unicast reply from 1.1.1.1 and 2.2.2.2) . Now from R2 when i ping 2.255.255.255( i receive a unicast reply from 2.2.2.2 ONLY) – fsociety Nov 22 '16 at 4:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.