Laptop computers can connect to each other directly by using an Ethernet cable. Apple’s MacBook Air, however, does not have an Ethernet port and so cannot be connected to another MacBook model directly in this manner. You can connect a MacBook Air to another MacBook using a USB cable by purchasing a USB-to-Ethernet adaptor. The USB-to-Ethernet adaptor’s USB cable will plug into the single USB port on the MacBook Air and allow for a connection to the other MacBook using Ethernet, even though the MacBook Air does not have an Ethernet port.
Stand the MacBook Air on a table. Open the MacBook Air’s lid to reveal the LCD screen. Rotate the MacBook Air so that the LCD screen is facing you.
Pull the port access door on the right side of the MacBook Air down with your fingers. Insert the USB cable from the USB-to-Ethernet adaptor into the USB slot that is closest to you inside of the port access door.
Stand the MacBook on the table to the left of the MacBook Air.
Plug one end of the Ethernet cable into the Ethernet port on the USB-to-Ethernet adaptor that is connected to the MacBook Air.
Plug the other end of the Ethernet cable into the Ethernet port on the side of the MacBook.
Third-party USB-to-Ethernet adaptors will work as well as the one that is made and sold by Apple.
You will need to configure both MacBooks to "see" each other with the “Network” control panel once the two are connected to each other.