Chapter 1 Internet Protocol Reference; Internet Protocol Header - Watchguard Firebox X1000 Reference Manual

Internet Protocol Reference
CHAPTER 1
Internet Protocol (IP) specifies the format of packets and the addressing
scheme for sending data over the Internet. By itself, it functions like a
postal system allowing you to address a package and drop it into the
system. There is, however, no direct link between you and the recipient.
In other words, there is no package.
Most networks combine IP with higher-level protocols like Transmission
Control Protocol (TCP). Unlike simple IP, TCP/IP establishes a
connection between two host servers so that they can send messages back
and forth. TCP/IP provides the "packaging."

Internet Protocol Header

IP is an Internet standard that enables the shipment of datagrams – self-
contained packets of information that include their own address and
delivery instructions. IP prepends a header to each datagram. The IP
header contains a minimum of twelve attributes as well as additional
optional attributes.
Reference Guide 1