A network mask is a numerical label used in computer networking to divide an IP address into two parts: the network portion and the host portion. It helps devices determine which part of the address identifies the specific network and which part identifies the individual device within that network. Network masks are often represented in a format similar to IP addresses, such as 255.255.255.0. In the context of the Internet, a network mask is crucial for routing data between different networks. It allows routers to efficiently direct traffic by identifying whether a destination IP address belongs to the same local network or a different one. This process is essential for maintaining effective communication across the Internet.