Three-Way Handshake

The three-way handshake is how two devices establish a TCP connection before exchanging data. First, the client sends a SYN (synchronize) packet. The server responds with SYN-ACK (synchronize-acknowledge). Finally, the client sends ACK (acknowledge) and the connection is established. This process confirms both sides are ready to communicate. When a port scan shows a port as "open," it means the three-way handshake completed successfully. If the server sends RST (reset) instead of SYN-ACK, the port is "closed." If there is no response at all, the port is "filtered."