1.) Simplified: The mask 255.0.0.0 means L3 devices will only care about the first octet of the IP address when routing the message (at least when determining which network ID the IP address is using), as only the first octet of the subnet has value, and it's maxed out (255 being the largest number possible for every octet).
This can be calculated by binary ANDing (though it isn't necessary in this case, and there's a far easier method for calculating this known as the "Magic Number").
But with ANDing, you convert the subnet to binary, in this case it is:
255.0.0.0 = 11111111.00000000.00000000.00000000
Then convert the IP address to binary, in this case, the server is:
10.4.2.251 = 00001010.00000100.00000010.11111011
Next you AND the two binary numbers (meaning if there's a '1' over a '1', you carry the '1' down. If there isn't a '1' over a '1', then you write a '0'.
Finally, you convert this binary number back to an IP address:
This proves that the subnet mask 255.0.0.0 means ONLY the first octet will be taken into account in determining what network an IP address is in.
2.) That being said, I think the first thing you should do is check to make sure all of your endpoints are using the same subnet mask (255.0.0.0, would work if you wanted all the addresses you listed to be able to communicate. 255.255.0.0 would also work with the addresses you gave. You could even do 255.255.128.0 or 255.255.192.0 or 255.255.224.0).