The difference is very, very slight. 9 times out of 10, they will mean the exact same thing.
However, in cases where we're discussing the subnetting of a given network, the two terms "network mask" and "sub-network mask" can have distinct meanings. That is, if we make a distinction between a "network" and a "sub-network" then "the mask of a network" and "the mask of a sub-network" mean different things. In all cases, this distinction is almost purely a relative distinction.
Let's say you've been issued the
10.10.0.0/16 network (using CIDR notation). Here, your "network mask" is
255.255.0.0. Let's say you need to separate this network into 4 smaller networks, each as large as they could possibly be. In order to get 4 networks out of
10.10.0.0/16, you need to borrow two bits (aka, decimal four) from the host address and use them for the subnet addresses. This will give you the following sub-networks:
Here, your "network mask" is still
255.255.0.0, but each "subnet mask" is
But, as I said, it's completely a relative term. One could also talk about
255.255.192.0 being a "network mask" and then
255.255.0.0 being a "supernet mask". For example,
10.10.0.0/16 is a supernet of, say,