A modem port connects an internal modem to a telephone line. A modem is
used to connect a computer to the Internet by making a data call over phone
lines to an Internet Service Provider (ISP).
Usually, there are two modem ports side by side. The one marked line (or with
a line symbol) is connected to the telephone line connector. The one marked
phone (or with a phone symbol) can optionally be connected to the telephone
handset if you want to use a telephone on the same connector.
If your computer does not have an internal modem installed, you will not see
these ports. You may also see an ethernet port, see "Ethernet Port" on
page 319. It looks similar to a modem port but is wider.