Fastnet is a proprietary network protocol developed by B&G for the exchange of boat data. It is physically based on the CAN layer and uses its differential data transmission. The actual overlying protocol is a serial protocol with 28800 baud, 8 data bits, two stop bits and one parity bit. One layer level higher, frames of different sizes are used. Some details on the protocol have been here and here collected.
Fastnet only works with devices from B&G and is not compatible with devices from other manufacturers. The use of the CAN layer probably only served to offer a competitor product with a similar name to NMEA2000 and CANet, which in reality is just a simple, slow serial protocol, but is significantly less powerful in terms of data transmission rate.
Fastnet is no longer relevant today and can only be found in a few old B&G installations.
There is one for Fastnet Open source implementation, which is implemented only very rudimentarily in C# for PC systems as a test program. A number of hardware interfaces to Fastnet can be found under the link.