With network_mode=true, both the following methods are possible
Case 1:
If the rbfeeder AND decoder (dump1090-fa OR dump1090-mutability, OR readsb) are on SAME RPi
[network]
mode=beast
external_port=30005
external_host=127.0.0.1
Case 2:
If the rbfeeder is on one RPi, and decoder (dump1090-fa OR dump1090-mutability, OR readsb) is on another RPi (but both RPi are on same local network), and assuming Local IP of RPi with dump1090 is 192.168.0.21
[network]
mode=beast
external_port=30005
external_host=192.168.0.21
.