What ethernet protocol? Are you rolling your own? Why use ethernet for 16-24 boards? Is it a requirement for the project? If so what else does the requirement state? Wired/wireless? gigabit? 10/100? Even before IoT there were a number of solutions (one uses a PIC even) now with IoT you have the ESP8266 as well as many others and they do a bunch of the work for you.
RS232 is an electrical and pin standard not a protocol standard, I assume you meant you were familiar with uart serial protocols instead of saying RS-232. Some of these parts support a uart interfaces, so it can really be a small matter of programming.
If you are not using a wiznet or esp8266 or other, then you might need to implement the stack. Doable in a PIC but I would be shopping for some other mcu to give you resource room and speed.
Depending on what you really need you could use a long distance uart solution (RS-422) and daisy chain, (or i2c or spi or mdio or can or other) you could use bluetooth, you could use infra red or IRDA, xbee/zigbee, etc.
Does everyone need to talk to everyone or is there one master and many slaves? What is the nature of the data, bursty, continuous, data rate, burst size, etc.
You are in no way ready to start talking about ethernet yet, you have a lot more reading to do. But before that you have to define your problem, full duplex, half duplex the nature of the data, continuous, bursty, unidirectional/bidirectional everyone sees everything or only master/slave, etc. That by itself may make the answer more obvious or at least reduce some that wont work. If someone gave you an "ethernet" requirement then there would have to be several other parameters than the word "ethernet" or "internet".