Ethernet ports are usually just relegated to connecting computers to routers to facilitate an internet connection, but they're capable of much more than that.
The sysadmin's guide to repurposing an RJ45 port ...
Some (many?) HDMI cables can also carry Ethernet over HDMI. The main use seems to be to provide internet connection to a smart TV from a cable modem box. Currently I have an ethernet-USB adaptor ...