Broadcasts with NAT

[chuck01]

I need to set up a NAT router with static translations of three addresses both ways. I also need broadcasts to be translated across. How does this work? Are they automatic, or can I add static entries for the broadcast addresses?

 

[rsivanandan]

First of all, the router won't pass the broadcast. Now if you want it to pass the broadcast to one specific host on the other side, you can use "helper address". For example, say I have a dhcp server 10.10.10.10 and I have dhcp clients on 172.0.0.1 interface of the router. Then you can tell the router to take the broadcast that will be produced on the 172 side and send it to the dhcp server directly (Unicast). What kind of broadcast are you looking for?

Cheers,
Rajesh

 

[chuck01]

Here's the situation- There are a set of machines that need to regularly switch between two discrete networks. They need both unicast and broadcast traffic from the network they are on at any time. We'd like to not have to change their IP address each time we swing them over, and could use a NAT router to translate the addresses when they're on the "other" network (the one that doesn't have their subnet). I understand how to place static entries for the addresses of the individual machines, but need to solve the broadcast problem. I can't find any reference to this in Cisco files, but hoped that by placing static entries for the broadcast addresses, we could get those through as well. Or is there another solution?

 

[rsivanandan]

We are looking at two issues here. As I said before; you can use the NAT on a router to have the machines behave as if they are on both networks.

But broadcast is a totally different idea. You can't correlate them because when you do NAT, you are natting the source address. But broadcast address would be the destination address. See the example below;

source is 10.1.1.1
destination is 172.1.1.1

I can do a nat here on 10.1.1.1 to represent as 172.1.1.2, natting is done on the source address.

On the other hand the broadcase scenario would be;

source is 10.1.1.1
destination 255.255.255.255

Router won't pass this traffic and natting is not related at all! If you need to pass the broadcast traffic along you will have to use a bridge. You can create a bridge group on the router and use it.

Cheers,
Rajesh

 

[chuck01]

Thanks. I understand all that, but the broadcasts I'm interested in are subnet-specific. So the source would be 192.168.4.255, and the dest. 192.168.1.255, for example, where both subnets were class C.
In this case could they be translated with static entries, or does this just not work?

 

[rsivanandan]

No Chuck, it won't work!

I understand what you are tried to tell me, but when a packet reaches a router which is configured for doing natting, the router treats the source address and broadcast address differently. The router simple hates the broadcast address. The only thing it can do is to take the broadcast traffic and send it to one specific host on the other side. Now if you put 1000 helper address on the interface, it may work, the router will take the broadcast message and send it to all the hosts on the other side as unicast message. Other than that there is no other way to do it through as you are thinking (NAT).

Cheers,
Rajesh