Question on Broadcasts

Hey everyone, I needed some helping figuring out something to do with broadcasts.

Here is what I am thinking of:

A router has 2 interfaces: Fa1 and Fa2.

Out Fa1 is the subnet 192.168.1.0/20
Out Fa2 is the subnet 192.168.1.16/20

Suppose PC1 is connected out Fa1 and wants to send a message to all the nodes out Fa2. The broadcast address out Fa2 is 192.168.1.31. However, Routers drop broadcasts. So how can PC1 send a message that hits all nodes on Fa2 if the router will drop messages addressed to 192.168.1.31 (since it is a broadcast address)?

Thank you.

Edit:

Also, I just thought of something else. If PC1 wanted to send a message to reach all nodes out Fa2, it would have to ARP for the MAC Address. But how can it ARP when the message is intended for multiple recipients?

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Comments

ColbyG

Each interface on a router is a broadcast domain. Broadcasts won't come in one interface of a router and leave another interface. You also can't have multiple routed interfaces in the same subnet.

Your networks are a little cooky btw.

typesh

ColbyNA wrote: »

You also can't have multiple routed interfaces in the same subnet.

Hey,

Not sure I understand...

These are different subnets ... the .0 subnet, and the .16 subnet. So for the purposes of routing, aren't they considered 2 different networks?

ColbyG

typesh wrote: »

Hey,

Not sure I understand...

These are different subnets ... the .0 subnet, and the .16 subnet. So for the purposes of routing, aren't they considered 2 different networks?

Oh, so then it looks like you meant to use a /28 mask, not a /20. In your post, those subnets are definitely the same network.

Broadcasts will not traverse a router. That should answer your question.

networker050184

typesh wrote: »

Hey,

Not sure I understand...

These are different subnets ... the .0 subnet, and the .16 subnet. So for the purposes of routing, aren't they considered 2 different networks?

No they are not two different subnets if you are using a /20 mask. I think you are looking for a /28.

Damn you Colby!

dynamik

Your subnet mask should be /28 though.

Edit: epic response time fellas!

ColbyG

I am the fastest! Haha.

typesh

Ahhhh! Can't believe I did that!

Yes, it should be a /28 mask!!

My apologies.

Lol thanks for the split-second responses!

ColbyNA wrote: »

Oh, so then it looks like you meant to use a /28 mask, not a /20. In your post, those subnets are definitely the same network.

Broadcasts will not traverse a router. That should answer your question.

Thanks! So then considering we are using a /28 mask, how can PC1 send a message to all the nodes out Fa2 since the broadcast won't traverse the router...

1number9

actually i think there is a way to set routers to forward broadcasts. i dont know how but i think i remember my Cisco 4 teacher saying it can be done.

nevermind I was probably thinking of forwarding DHCP broadcasts.

ColbyG

1number9 wrote: »

actually i think there is a way to set routers to forward broadcasts. i dont know how but i think i remember my Cisco 4 teacher saying it can be done.

nevermind I was probably thinking of forwarding DHCP broadcasts.

You can forward some broadcasts with the ip-helper command.

typesh

Hey thanks for the reply. I figured out the ARP part. Stupid mistake. If sending a message to someone outside your subnet, then the PC would ARP for the default-gateway's MAC.

So if PC1 wanted a message to get to all nodes on Fa2, the the ip helper command would be the only way to allow this?

networker050184

typesh wrote: »

Hey thanks for the reply. I figured out the ARP part. Stupid mistake. If sending a message to someone outside your subnet, then the PC would ARP for the default-gateway's MAC.

So if PC1 wanted a message to get to all nodes on Fa2, the the ip helper command would be the only way to allow this?

You probably are looking for something along the lines of multicast. Is there a certain scenario you are trying to figure out or is this just a general questions? We might be able to help you out with something specific.

mikej412

If your router is configured to allow ip directed broadcasts, you may be able to send the message to the IP subnet broadcast address.

dynamik

Are you thinking of ip helper-address?

typesh

Hey thanks for the responses.

networker050184:

It's not really something specific I am working on... I am just thinking about scenarios-- walking through routing logic/switching logic in my mind. I was thinking about LAN broadcasts where a host sends out a broadcast which reaches everyone on its own subnet. Then I started thinking "what if this host wanted to reach everyone on a different subnet". I couldn't reason it out in my head. Maybe this is not a practical situation in real life. I haven't come across any examples of it in my books (that I can remember).

Heero

2 options

1. Put the PC on the other subnet.
2. Implement a multicast configuration. Multicast was developed for just this kind of situation.

The ip-helper address is able to take a broadcast and forward it as a unicast to a specific host on another subnet, not really viable for a large number of recipeints.