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IPv4 Datagram Fields

If you are pursuing CompTIA Network+ certification, of if you're involved in network troubleshooting, it's important to understand the construction of an IP datagram. IPv4 is the most commonly used IP addressing format today, but because the pool of available IPv4 addresses is rapidly becoming exhausted, the Internet is beginning to change to the IPv6 format.

Sometimes an IP datagram will be called a packet because in it's route from source to destination it can be switched from one path to another depending upon which path is best ,in a process called packet switching. Sometimes an IP datagram will be called a frame because of its position in the OSI networking model.

Shown below is the format and meaning of the fields in an IPv4 datagram, as defined by RFC 0791. Everything above the data is called the header. The information in the header determines where the data (sometimes called the payload) goes and how its interpreted.

IPv4 Datagram Fields
IPv4 Datagram Fields

Version: 4 bit field that defines the version to which IP packet belongs (IPv4 or IPv6).

Header Length: 4-bit field that defines the header length. A header can vary in length because of Option field.

Type of Service: 8-bit field is used to carry Quality of Service information.

Total Length: 16-bit field that defines the total length of the IP datagram including the header and the data. Because it's a 16-bit field, the biggest number it can hold is 65,535, so the maximum size of an IP datagram can't be more than 65,535 bytes.

Identifier: 16-bit field that identifies a fragment. As a packet travels from source to destination, it may come upon circuits who's Maximum Transfer Unit (MTU) size is smaller than the IP packet's size. The packet must then be divided into fragments and each fragment needs to be marked with the same identifier.

Flags: 3-bit field that defines how many fragments remain in a series of fragments.

Fragment Offset: 13-bit field that defines where in the overall message each particular fragment should be placed.

Time to Live: 8-bit field that is set to the maximum number of routers the packet can pass through, called hops, before the packet is dropped (usually set to 255).

Protocol: 8-bit field that identifies the higher-layer protocol (usually a Transport layer protocol) carried in the datagram.

Header Checksum: 16-bit field that contains a checksum used for detecting errors in the IP header.

Source Address: 32-bit field that contains the packets source IP address.

Destination Address: 32-bit field that contains the packets destination IP address.

Options: an optional variable length field that is rarely used, but may be used for purposes of debugging or gathering statistical information. If the options field is not 32-bits, a special Padding field will be added to make sure the size of the header remains a multiple of 32 bits.

Data: a variable length field sometimes referred to as the payload that consists of datagrams of upper-layer protocols such as TCP, UDP, and ICMP.

More Networking Protocols and Standards:
• IPv6 Payload Length Field and Jumbograms
• Basic TCP/IP Networking
• Comparing IPv4 and IPv6 at a Glance
• TCP Windowing
• Free eBook: IPv6 Addressing
• OSI Network Model
• An Introduction to the Types of VPNs
• RADIUS Protocol
• Wireless Standards - 802.11a 802.11b 802.11g 802.11n 802.11i Explained
• A Simple Description of the IPv6 Header and Datagram

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