Again, you almost always can assume the subnet or gateway, but it shouldn't be coded
The default netmask is defined by the IPv4 standard. It can and should be coded. Every single network capable operating system I have ever encountered allows one to use the default subnet mask. Why would you remove this from the arduino?
You're on a network that does not use a default netmask; big deal. I've been on a network where the netmask changes depending on which subnet you're on. If the user is on such a network they will likely be assigned an address anyway (which will come with subnet and router data!), instead of being allowed to run amok and chose an IP configuration by rectal extraction.
The simple fact is that most arduino users will be on a network where the default netmask is correct and the default router is .1. The arduino is meant to be easy to use. connect the dots...
-j