the main diffrense between subnet mask and default sunet 
mask is that a subnet mask tell about the from which calss 
any network ip address is belonged.where as a default 
sumnet mask doesnot difine the the claases of ip address it 
only define the route of a prtocall

a  subnet mask can be changed as per the hosts/subnets requirement.default subnet mask is whats been the default for a particular  address class(class C=255.255.255.0, class B=255.255.0.0 class A = 255.0.0.0).
but we can use a class C address with subnet 255.255.0.0 i.e 192.168.1.2/16 if we need more hosts.
hope this helps

The default subnet mask just inbuild network portion(bits)
which is already define.in ipv4 class A has default subnet 
mask is 255.0.0.0,for Class B =255.255.0.0,and for class C 
= 255.255.255.0, where as subnet mask represent the number 
of bits used by network and rest for Host(after 
subnetting).if there is no subnetting occur in network then 
the both are same.