I'm baffled as to why the author is using STM32F401 with a W5100 Ethernet shield when they could just as easily use an STM32F407 board, which includes a built-in Ethernet MAC, coupled with a cheap (2 for ~$12) Ethernet PHY board. Lots of example Ethernet projects available and just as easy to develop for as the STM32F401.
Also,
> Due to the complexity of the signalling involved with ethernet, a dedicated ASIC is generally used
In the context of microcontrollers, I think this is generally not true. In most cases Ethernet functionality is incorporated as a embedded peripheral within the microcontroller itself, as it is with the STM32F407 and the ESP32 (both of which happen to use the same Ethernet peripheral sourced from an external provider).
Also,
> Due to the complexity of the signalling involved with ethernet, a dedicated ASIC is generally used
In the context of microcontrollers, I think this is generally not true. In most cases Ethernet functionality is incorporated as a embedded peripheral within the microcontroller itself, as it is with the STM32F407 and the ESP32 (both of which happen to use the same Ethernet peripheral sourced from an external provider).