I think it would be better to talk about the ports before we proceed any further. A port is a special memory location that exists when two computers are in communication via TCP/IP. Applications use a port number as an identifier to other computers, both the sending and receiving computers use this port to exchange data.
To make the job of communication easier, some port numbers have been standardized. These standard port numbers have no inherent value other than that users have agreed to use them with certain applications. Table below lists a number of popular and publicly accepted port numbers and their corresponding applications.
| Service | Port |
| HTTP | 80 |
| FTP | 20,21 |
| Gopher | 70 |
| SMTP | 25 |
| POP3 | 110 |
| Telnet | 23 |
| Finger | 79 |
| Local loops/callbacks | 0 |
Using the Winsock Control
Winsock is above the TCP/IP protocol stack in the ISO/OSI model. TCP/IP is an industry standard communication protocol that defines methods for packaging data into packets for transmission between computing devices on a heterogeneous network. TCP/IP is the standard for data transmission over networks, including the Internet. TCP establishes a connection for data transmission and IP defines the method for sending data packets.
The Microsoft Winsock control makes using the TCP/IP a breeze. Microsoft has wrapped up the Winsock and INetAPI API calls into a nice neat package that you can easily incorporate into your Visual Basic applications.
Winsock Operating Modes
The Transport layer (also known as the Host-to-Host Transport layer) is responsible for providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are TCP and User Datagram Protocol (UDP). The Winsock control supports the following two operating modes:
- sckTCPProtocol
- sckUDPProtocol
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