Re: [lwip-users] Sending to broadcast address 192.168.2.255

[David Empson]

----- Original Message ----- 
From: "Marco Jakobs" <address@hidden>
To: "Mailing list for lwIP users" <address@hidden>
Sent: Thursday, August 26, 2010 8:04 AM
Subject: Re: [lwip-users] Sending to broadcast address 192.168.2.255


> Hi Kieran,
>
> my netmask is a bit more extended, it's set to 255.255.240.0 as the PC's 
> are in a different subnet and they must be able to access the devices by 
> telnet, web and TFTP. So the decision for a broadcast is determined by the 
> netmask and an octet of "255" and the end does not mean a broadcast to the 
> subnet?

No. To broadcast to the subnet, your address must contain 1s in all bits 
corresponding to 0 bits in the subnet mask, and match the network address in 
all bits corresponding to 1 bits in the subnet mask.

In this case your subnet mask is 255.255.240.0, so your network is 
192.168.0/20 using CIDR notation (20 bit network, 12 bit machine).

All addresses from 192.168.0.1 through 192.168.15.254 are specific machine 
addresses within your network, including 192.168.2.255. You cannot broadcast 
to part of the subnet, only the entire subnet. The correct subnet broadcast 
address in this case would be 192.168.15.255.

Every machine involved in the subnet must be using the same subnet mask in 
order to correctly interpret the subnet broadcast address.

> The broadcast packets i've seen on the PC with the same netmask were 
> caused by a different behaviour of windows: It also ARPs (i've not seen 
> this in the previous trace), and then send it as a broadcast after the 
> ARPs timed out. Not that good idea for LwIP as the packets must all be 
> buffered until the ARP has timed out ...

Was the PC trying to broadcast on 192.168.2.255 or 192.168.15.255?

> So i've changed it back to a general broadcast in my project ;-)

That's certainly easier.