Exploring the Different Types of Network TAPs
https://insights.profitap.com/exploring-the-different-types-of-network-taps?utm_campaign=Datacenter%20%5BFiber%20%2F%20Copper%5D&utm_content=137597877&utm_medium=social&utm_source=facebook&hss_channel=fbp-937557376324166

The Great Scott Gadgets Throwing Star LAN Tap is a small, simple device for monitoring Ethernet communications. It is available in the original kit form

[Kurt] likes to know what’s going on with his network. He already uses bandwidth checking software on his DD-WRT capable router, but he wanted a second opinion. So he built his own network monitor. [Kurt] started by building a passive Ethernet tap. He then needed a network interface chip that would serve his purposes. The common Wiznet chips used with Arduinos didn’t allow enough manipulation of raw packet data, so he switched to a Microchip ENC624J600 (PDF). The Microchip controller allowed him to count the bytes in the raw Ethernet packets.

Throwing Star LAN Tap
http://ossmann.blogspot.com/2011/02/throwing-star-lan-tap.html

The basic implementation is similar as my drawing but implemented with circuit board.

The trick is that ach of those extra pairs not needed for 10/100 Mbit/s Ethernet are bypassed with a 220 pF capacitor. his filters out the high frequency signals of 1000BASE-T, forcing the target devices to revert to 100BASE-TX which can then be monitored. The capacitors don’t adversely affect lower frequency RS-232 signals, so all eight conductors function when monitoring serial connections. Sure, it’s an ugly hack, but it’s an ugly hack that fits in your pocket.