WIZnet Developer Forum

UDP broadcasts and netmask

I have a network with 6 W5100s set as follows:

netmask for everything is, sends packets to

and, sends packets to

These should be two independent IP subnets, though they share the same physical ethernet.

However, if sends a broadcast to, it is received not only by and 102,
but also by ! (I have not checked on 103 and 104).

Is this a bug or does someone know why this is not being filtered out by the netmask on


I will test to receive broadcast packet as same your situation and then let you know the test result.

Thank you.


I have done more testing and have more info, as the problem seems to only occur under certain conditions:

Setup is netmask on all devices (all W5100) sends direct UDP data to and
also sometimes it sends broadcasts to and sometimes only to

if another W5100, address is listening for UDP packets on same port, it sees the broadcasts but not the broadcast, and this is correct. is also sending direct packets to two other W5100, and (these only listen for packets and I think not relevant).

However, if now sends a broadcast to, then it starts receiving broadcasts for! it seems like sending a broadcast packet breaks the incoming filter.

if need more info let me know, thanks!


I tested about your question using W5100E01-AVR(firmware v1.4.5).
So far it’s been no problem.
If possible, can you upload your driver’s source code onto here(related send function)?

Are you using the our firmware?

I hope to hear your prompt response.

thanks :slight_smile:

Hi, Mickle.

I’m sorry that I forgot your problem.

How about your problem.
Can you tell me your status?

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