At the core of network application development is writing programs that run on different end systems and communicate
with each other over the network. In the Web application there are two distinct programs that communicate with each
other: the browser program running in the user's host; and the Web server prgram running in the Web server host.
When developing your new application, you need to write software that will run on multiple machines. Importantly,
you do not need ot write software that runs on network-core devices, such as routers or Ethernet switches.
Network-core devices do not function at the application layer; they function at lower layers, specifically at the network
layer and below.
