1. The first packet sent across a TCP connection is known as a "SYN" or "synchronize" packet. For example, when you contact http://www.robertgraham.com, the first packet your systems out will be a SYN packet to the HTTP port 80 on www.robertgraham.com. Your browser is telling the web server that it wants to connect. Key point: Most packet-filtering firewalls work by blocking the SYN packets. This stops connections from being initiated. You can still scan behind these firewalls using ACK or FIN packets, but you will not be able to connect to any of those machines. See also: SYN flood, three-way-handshake, TCP From Hacking-Lexicon