Data travels along the Internet
in packets that are sent individually across the network and then reassembled
into the original data at the correct recipient address. Each packet is like a
letter in that it has a sender and a receiver. When the packet reaches the
correct receiver address, it stops traveling.
Every packet has the following fields: source IP address (such as 10.23.1.156); destination
IP address; transport type (such as ICMP=1,
TCP=6, UDP=17); source port and destination port (such as DNS=53, FTP=21, HTTP=80); and flags (such as SYN).
Encapsulation; Internet; Internet Protocol (IP); IP Address; Port and Port
Numbers; Synchronize Packet (SYN).
Graham, R. Hacking Lexicon. [Online, 2001.] Robert Graham Website.