How you can identify there is a connection between two networks?

Answer 1

A network A is connected to a network B if any networked device within network A can reach some device within network B. The question that is more significant is, "How can we tell if two computers are connected over a network". Let's answer this question with respect to UDP and TCP (two methods of network communication between hosts). In order for a connective or connectionless connection to be made between two computers, they must be running the same protocol, one or both devices involved must have either other's IP (internet Protocol) addresses, and (at least) one machine must know the port that the other machine is using to run the network protocol.

For UDP, no connection is actually established, even though a connection pipe may be available. Usually with UDP, a server runs an application which implements UDP on a specific port and clients listen over that port. Despite the fact that there is no acknowledgement between the client and server, data is transfered.

TCP is similar except a 'handshake' occurs where two machines actually bind themselves to each other using a socket. Both the client and the server agree on ports to use to establish a connection.

You can use applications like 'netstat' or 'nmap' to see what ports are being used (and what type of protocol is being used on that port). In addition, you can use a tool like 'wireshark' to see the actual TCP and UDP packet streams occurring over these ports. These TCP and UDP streams may be encapsulated within a higher level protocol like HTTP, SSH, FTP, or several other popular services.