Layer 4, transport: The transport layer is responsible for taking the chunk
of data from the application and preparing it for shipment onto the network.
Prepping data for transport involves chopping the chunk into smaller
pieces and adding a header that identifies the sending and receiving application
(otherwise known as port numbers). For example, Hypertext Transfer
Protocol (HTTP) web traffic uses port 80, and FTP traffic uses port 21.
Each piece of data and its associated headers is called a packet.