Protocol stack software is a set of hierarchical network protocols that manage the flow of data in communication systems. Each protocol in the stack or suite supports the protocol above it and uses the one below it. Protocol stacks can be divided into application, transport, and physical categories (or layers). For example, in a Win32 computer system, the Winsock (Windows socket) dynamic link library (DLL) implements the transmission control protocol/Internet protocol (TCP/IP) from the network interface card (NIC) to an application such as a Web browser or e-mail client. This protocol is carried across a transport mechanism such as a Cat5e network cable. Most protocol stacks operate within the open system interconnection (OSI) reference model of communications. The TCP/IP protocol stack is a common example of a stack that uses protocol stack software.