I would comment on @m-rostami's post but I don't have sufficient reputation yet.
The EtherCAT shields are very expensive, but adding USB->Ethernet adapters will work just fine. Yes, the performance will be very poor, but you probably don't care about that because you are running this as a learning experiment.
Remember that the Raspberry Pi model 3 and earlier only supports USB-2 and your network performance will be poor anyway because you will have lots of adapters sharing the same USB bus, so there is no point buying more expensive USB3 or GbE adpaters if you're on a model 3. Just buy the cheapest. I've used lots of different versions for various reasons and they all work fine if you install the drivers. That having been said, the older USB2->100 BaseT adapters are getting harder to find now because people have moved over to USB3->GbE.
Adapters that I can recommend as I personally tried them and found them to work well are:
Manufacturer Model Style Driver
Anker AK-A7610011 USB3->GbE r8152
Belkin B2B048 USB3->GbE r8152
CableMatters 202023-BLK USB2->10/100 r8152, smsc95xx
CableMatters 202013-BLK UBS3->GbE r8152
Linksys USB3GIG-EJ USB3->GbE r8152
Trendnet TU3-ETG USB3->GbE ax88179_178a
Just remember that they all need power so get a big power supply for your Pi or, better still, use a powered external USB hub. The power supply design in the Pi is very "cost optimised" and doesn't work well if you load it up with lots of power-hungry peripherals, and USB->Ethernet adapters (especially GbE ones) can draw quite a bit of power. Using a powered external USB hub completely solves the power issues.