At the next layer, a set of routines defines how data packets will operate under the rules of specific network protocols like TCP/IP, IPX or NetBIOS. At this layer, network-layer constructs, such as TCP and User Datagram Protocol (UDP) sockets, are defined to the operating system. The TCP/IP kernel software ultimately delivers a complete set of network capabilities through a standard programming interface.

Network software is wri tten in logical layers. The TCP/IP kernel operates between the network hardware and the network application software. The lower layer of the stack interfaces with device drivers for the network hardware (Ethernet card, ISDN card or modem). TCP/IP kernels must be designed to share network hardware with other protocols, such as IPX and NetBIOS, using multiprotocol interfaces like NDIS or ODI. The higher layer of the stack takes the form of a programming interface that services network requests from applications. The top and bottom of the stack are defined by the requirements of the hardware and software interfaces. The kernel itself is proprietary software that must implement the protocols that connect these two interfaces (see "TCP/IP Network Layers," on page 184).

In the Windows environment, the Windows Sockets application programming interface (API) lies at the top of the TCP/IP kernel and performs the network-related tasks of higher-level application programs. WinSock 1.1 has long been the standard for integrating TCP/IP into the various Windows flavors. WinSock 2.0 extends the capabilities of the earlier version by enhancing the features available for TCP/IP and by supporting other network protocols, such as IPX. Look for WinSock 2.0 support if your organization develops multiprotocol network software or if it's considering implementing applications based on this version of WinSock.

TCP/IP kernel features include ease of installation, configuration and administration. When considering TCP/IP products, look for support of BOOTP or DHCP for automatic configuration because few system administrators have time to manually configure every workstation. Also, the TCP/IP kernel must be tightly integrated into the operating system to achieve stability and high performance. Don't be satisfied with a sluggish network kernel or one that causes the operating system to crash.

TCP/IP-Application Suites You need application programs to take advantage of a TCP/IP network. A package of TCP/IP programs and uti lities, or application suites, forms the more tangible aspect of TCP/IP network software, in contrast to the kernel, which operates out of sight. Programs within a TCP/IP suite range from command-line utilities to graphical clients. A well-designed application suite integrates a collection of programs that blends with similar user interfaces. The following applications are typically included in TCP/IP application suites.

· Remote login Telnet and rlogin let you connect to a remote computer and execute commands and programs. Telnet applications have become relatively sophisticated programs. Host-based software often depends on terminal-emulation features, such as cursor and font control, graphics, complex command sets and other routines. Run-of-the-mill telnet software implements the terminal characteristics of Digital Equipment Corp.'s VT-220, but others offer more advanced capabilities. Some telnet clients emulate IBM's 3270- and 5250-style terminals and Tektr onix's graphics terminals. Telnet clien ts can use programming interfaces to intercept data streams from the terminal and channel them into other applications.

· File transfer FTP and TFTP transfer data among computers on a TCP/IP network. Look for an FTP client that follows a point-and-click or drag-and-drop interface modeled after the file browsers that are integrated into the desktop operating system, rather than one based on Unix-like commands.

· Network printing Network printing is a traditional TCP/IP applications suite feature. TCP/IP networks can share printers through the Unix-like line printer protocol (lp and lpr) or through print services associated with Network File System (NFS). However, the prevalence of network-ready printers and built-in support for network printing lessens the need for TCP/IP-protocol usage for sharing printers.