A PRI line can have only one device connected. This device will have a single, predefined address which, in practice, is always zero. When a ISPBX is attached to the PRI line, it will perform its own call management and call routing functions for the devices attached to it.

A BRI line can have up to eight devices connected to it. Device addresses may be pre-configured or dynamically allocated.

• In dynamic addressing, each device has to request an address from the network before it can perform any signalling activity.
• With fixed addressing, the address of each device has to be configured to match the addresses pre-configured for this line.

ISDN service suppliers do not allow both fixed and dynamic addressing on the same line. In practice, an ISDN line where the device addresses are fixed will only permit a single device. The is no convention to describe this type of line configuration, but ISDN service providers sometimes give this a name that reflects this type of configuration. See 'Practical Applications' at the end of this section for examples.