Data link control (DLC) is a transport protocol defined by IBM. It is mainly used to communicate with IBM mainframes and minicomputers, typically model 3270 or AS/400 machines. In addition, Windows NT also uses DLC to communicate with network printers such as the Hewlett-Packard (HP) LaserJet 4 Si.

DLC works with Token Ring (802.5) or Ethernet (802.3) NDIS (network driver interface specification) MAC (media access control) drivers.

DLC is available to 32-bit Windows NT-based programs and 16-bit MS-DOS-based and Windows-based programs. Since there are few 32-bit 3270 emulators available, DLC is typically by 16-bit 3270 and 5250 (AS/400) emulators. These are character-mode or graphics-mode MS-DOS-based programs or Windows-based programs that communicate with an IBM mainframe using 3270 terminal emulation.

The following 16-bit 3270 and 5250 emulation programs should work normally with Windows NT:

For IBM 3270: For IBM 5250: Windows NT also uses DLC to communicate with network printers, such as the HP LaserJet 4 Si. Such printers have a network interface card (NIC) that contains a MAC and a DLC protocol stack. Print jobs are sent to these printers as DLC-level frames over the network, instead of over the parallel port. The advantages of such printer setups are increased print job download speed and the ability to have the printer physically distant from its server. Note that even though the printer has the ability to receive print jobs over the network, with Windows NT, it is usually controlled by a print server that has sole control of the network connection to the printer (meaning that other computers cannot connect to the printer, even if they know the network address of the printer). This is because DLC uses service access points (SAPs) and only one is available for connections. Once a remote computer has connected to the SAP, subsequent connect requests are denied at the printer.