A few manufacturers have hardware that allows you to connect computers on two different lines within the same home. A phone line can carry multiple signals at different frequencies which is why you can talk on the phone and still use a DSL modem for internet access. Network traffic over phone lines use frequencies higher than DSL or voice bands. Nearly all homes can use HomePNA 1.0 networks, but low-quality wiring in older homes may interfere with version 2.0 networks. If you already have an ethernet card with a standard connector, adapters are available that convert it for use with RJ-11 phone cord.
Laptops can use PC Card adapters with phone cord connectors.
Phone line NICs are installed into an empty expansion slot with the phone cord plugging directly into the card. As long as all your network hardware is HomePNA compatible, you can mix and match within a network. So you can have an older computer with a parallel port adapter and a newer one with an ethernet adapter. HomePNA products from different manufacturers should be compatible so cross-platform sharing is possible with the right software. Even HomePNA 2.0 networks are not as fast as a traditional 10 Mbps ethernet.