I wanted to implement a ethernet interface in ESP8266 module.
Just in time, Espressif released open-source-lwIP as below.
So, for just test reason, I implemented ethernet packet handling code using ESP8266 as above.
With this test, I'm sure that many projects with ESP8266 and ethernet are possible.
Ethernet to WiFi (Ethernet to 802.11 converter)
Dual NIC Application(WiFi and Ethernet)
And, I think that WIZnet's W5500 is a best choice for the ESP8266's ethernet interface.
Please refer to my test result and source codes as below.