Dell Inspiron 4100 - Manual (Page 68)

Infrared Sensor
The infrared sensor lets you transfer files from your computer to another infrared-compatible device without using cable connections.
Audio Connectors
Attach headphones or speakers to the connector.
Attach a microphone to the connector.
Modem Connector
To use your computer's internal modem, connect the telephone cable to the modem connector.
For information on using the modem, see the online modem documentation supplied with your computer.
Network Connector
To use your computer's integrated network adapter, use this connector to connect the computer to a network.