Where is martian from?

Pls assist me to define martian source.
Here is log message:

martian source 255.255.255.255 from 1.2.3.4, on dev eth1

Here is my network design.
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I have W2K with IP adresses:
eth0 Ext IP – 1.2.3.4/24
eth1 Int IP – 5.6.7.8/24

I have Linux box with IP address:
eth0 Ext IP – 1.2.3.5/24
eth1 Int IP – 5.6.7.9/24
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This is not permanent message in my log, however I want to understand what I done wrong. Maybe I need to disable some services on W2K?

Any suggestion welcome.

TIA