Hi,
sorry for the late reply. What you are describing is exactly what’s implemented in the first tutorial: one physical network interface (eth0, as it all began from one rpi3b+ physical limitations) and not wanting to add usb network interfaces, nor use the wlan interface. You can create virtual network interfaces that are tagged with defined ids and that work at layer 3 level, having a unique ip and all other network configuration (you can also have multiple ips configured to a unique interface in linux with aliases, but they will travel the same “pipe”, so only with proper subnet masking could you segregate these interfaces from “seeing” each other. A bit more like a security through obscurity kind of thing. By adding vlans on top of that physical hardware, you are fully segregating networks.
Something like:
#nmcli con add type vlan con-name eth0@vlan3 dev eth0 id 3 ip4 "your_vlan3_ip_here/your_vlan3_subnet_mask_here" ipv4.dns "your_vlan3_dns_ip_here" gw4 "your_vlan3_gateway_here"
Hope this helps 