The 21st Century Network (21CN) programme is the network transformation project of the UK telecommunications company BT Group plc for data and voice. It had been intended to move BT's telephone network from the present AXE/System X Public Switched Telephone Network (PSTN) to an Internet Protocol (IP) system. As well as switching over the PSTN, BT planned to deliver many additional services over their new data network, such as on-demand interactive TV services.

BT originally stated that it would accrue annual savings of £1 billion when the transition to the new network was completed, and hoped to have over 50% of its customers transferred by 2008. (See External links below for current progress.) Capital expenditure was put at £10 billion over five years, this being 75% of BT's total capital spending plans in that period.

The new network is based on an architectural model of five classes of network nodes. These are:

Premises nodes includes residential, Small-Medium Enterprise (SME) and Enterprise. The presumption is that all these will have high speed connections to the network, delivered over copper (in the form of ADSL or other DSL technologies) or over fibre, as either PON or direct fibre in the case of large enterprises. The sole exception to this presumption is for legacy PSTN, where provision will continue to be made for analogue voice.