KallistiOS (the Dreamcast homebrew SDK) supports most of the standard sockets API used by pretty much every OS for developing networked code. Pretty much anyone who has developed a C program using the standard sockets API on a PC should be able to figure it out.
You certainly don't need to worry about the low level details of dealing with the hardware of the modem and/or BBA -- that's all already handled internally in KallistiOS.