Direct Dialling Inward. DID. The actual telephone number associated with an extension or handset.

Dynamic Host Configuration Protocol; sometimes Dynamic Host Control Protocol. A system that automatically distributes IP addresses to devices on a network.

Direct Inward Dialling; also DDI. The actual telephone number associated with an extension or handset.

Do Not Disturb. A feature on a telephone handset that effectively puts it into standby.

Domain Name Service. A system that associates URLs with IP addresses e.g. it is convenient for users to type bbc.co.uk to access the BBC website, but the web browser needs to know that the address might be 212.58.244.23. DNS provides the browser with this association.

DTMF (Dual-tone multi-frequency signalling) is an in-band telecommunication signalling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centres. The Touch-Tone system (USA) using a telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline and mobile service. Other multi-frequency systems are used for internal signalling within the telephone network.

In relation to computer networks, dynamic can refer to IP addresses that are assigned to a device automatically (DHCP) and that can change after a period of time - from the next boot to never. It can also refer to ’dynamic routing’ - a general term for a number of protocols that enable routers to learn for themselves the best route for data at any one time. The alternatives are static IP addressing and static routing.