What is the netmask value?

Could you please elaborate on the concept of a netmask value in the context of networking and specifically, how it relates to IP addressing? I'm curious to understand its significance and how it's utilized to segment and categorize IP addresses within a network. Specifically, what role does the netmask play in determining which parts of an IP address represent the network ID and which represent the host ID? I'd appreciate a concise yet comprehensive explanation of the netmask's purpose and function in network design and administration.