What is an example of a network mask?

Could you elaborate on the concept of a network mask, perhaps with a specific example? As a person seeking to gain deeper understanding in the realm of networking and cryptography, I'm curious to know how a network mask functions in practice. For instance, if we consider a typical IPv4 address, how would a network mask be applied to distinguish between the network portion and the host portion of the address? An illustrative example with actual numbers would be greatly appreciated.