In general, small businesses are comprised of a single network that uses a single network address and mask (e.g., the network address 192.168.1.0 255.255.255.0 used throughout the examples in this book). However, large companies or companies that have multiple departments often use multiple networks or break up their network into smaller network segments called subnetworks (subnets). Either networking method provides each department its own LAN within the same company network. Cisco refers to this network design as an “internetwork,” hence the reason the IOS is called an Internetworking Operating System. Using routers and routing protocols, a company can control the flow of data between departments within the company internetwork (simply called a “network” by most IT professionals).