To transfer data between two computers, we need a device to convert a stream of bytes into a signal matching the medium that will be used for the transfer. That device is the network card or network adapter.
A network card is connected on one end with the data bus on the motherboard and on the other to the physical medium - for example, to a network cable.
The connection between the network adapter and the cable is called a port. A network adapter may have one or more ports (and are typically numbered from 1 to n; where n is the number of ports).
Before the data is converted to the transfer signal, it is split into smaller parts - where each part gets tagged with meta data describing where it is coming from and where it is heading - along with other information. This new combination is called a data frame or a data packet. This is the unit of sending and receiving in computer networking.
Then, the network adapter converts that stream of bytes to the transfer signal on the transfer medium. A receiver network adapter converts the signal back to a stream of bytes matching the original.
Typically, there is a protocol for the network cards sharing the same physical medium that orchestrates when each network card can place its signal on the medium. This orchestration is beyond the scope of this article series though.
In the above very simple topology, if Network Card A receives data, it must be coming from Network Card B. If it sends data, it must be going to Network Card B. The notion of sender/receiver identity is implied by the physical wiring.
But what happens when we want to connect a third computer?
Let's see how this will look like in our next article!