title: How can server application check if a client application is online?
body: <p>I have a server application which monitors the clients' activities in the network (all codes are in C# winforms). The server application already has IP addresses of client computers. I want to know how to check </p>
  
  <p>i) if the clients are online (i.e, have opened and been using the client applications) or offline (not using the applications)?</p>
  
  <p>ii) if the clients' computers are opened?</p>
  
  <p>I'm not good in networking and have not got it after searching for hours on internet. Please help!</p>
  
  <p>Edited: I have seen the mentions of HTTP, TCP, UDP, etc. in various sites about server-client applications. What are the differences between them and which one should be used for the above?</p>
  
  <p>PS: Sorry for extra questions. The questions are related so I dont want to make a new post.</p>