IP Datagram Header

Notes:

The heart of the TCP/IP protocol suite is IP, the Internet Protocol, described in RFC 791, IP provides a connectionless packet delivery services. All other network services in the TCP/IP suite are built on top of this service. The main job of IP is to forward packets to their destination.
IP defines the datagram as the basic unit of transmission across TCP/IP. A datagram is a piece of data with control information, such as source and destination addresses, attached. If a datagram is fragmented en-route, the receiver must reconstruct the datagram before passing it up the protocol stack. Fragmented packets are more difficult for packet filters (the basis of a firewall) to process, more on this later.
Each IP packet has a header attached as shown in the slide. Some of the more important fields in the header are:
SOURCE IP ADDRESS The IP address of the host that originated the packet. Do not rely on the accuracy of this value.
DESTINATION IP ADDRESS The IP address of the host to whom this packet is ultimately destined: it is not the address of the next router along the way; routers primarily receive packets that are actually addressed to someone else.
FLAGS and FRAGMENT OFFSET Used to control fragmentation and reassembly.
PROTOCOL Indicates which transport protocol is to receive the packet at the destination.
CHECKSUM, note: only applies to the header.
DATA, this is readable by any host connected to a network used in routing the data to its destination.
IP has a number of optional fields but they are not commonly used. These cover areas such as security and strict and loose routing. IP security used mainly in military sites where each packet is classified by the sensitivity of the data it carries and can only use certain routes and be read by certain applications.