An IP version 4 header contains 14 fields and is 24 bytes long. It has enough information to deliver the packet to its destination, but if something goes wrong the routers cannot detect or fix the problem. Some of the more interesting fields are:
| Name | Size (bits) |
Meaning |
|---|---|---|
| Version | 4 | The IP version number (4 in this case) |
| Time to live | 8 | The number of router hops left before the packet is discarded |
| Source address | 32 | The IP address of the host that sent the packet |
| Destination address | 32 | The IP address of the host the packet is being routed to |
| Packet length | 16 | The total length of the packet (the header plus the payload) in bytes |
A TCP header contains 12 fields and is 24 bytes long. The Port field identifies the applications (processes) running on the computers at both ends of the conversation. The sequence number and checksum allow for the detection of lost or corrupted packets or those which arrive out of order. Some of the more interesting fields are:
| Name | Size (bits) |
Meaning |
|---|---|---|
| Source port | 16 | Identifies the sending application program |
| Destination port | 16 | Identifies the receiving application program |
| Sequence number | 32 | Used for ordering segments |
| Check sum | 16 | Used to detect transmission errors |
A UDP header contains 4 fields and is 8 bytes long. Like TCP, UDP is used for transport. UDP is simpler and faster, and is used in situations where error checking and session management are not needed. The fields are:
| Name | Size (bits) |
Meaning |
|---|---|---|
| Source port | 16 | Identifies the sending application program |
| Destination port | 16 | Identifies the receiving application program |
| Length | 16 | Datagram length |
| Check sum | 16 | Optional |
An IP version 6 header contains 8 fields and is 40 bytes long. The packet structure is simpler than IP version 4, but the long address field allows for an astronomical number of unique IP address and the Priority and Flow label fields can be used to increase routing efficiency and quality of service. IP version 6 is just beginning to be deployed. Some of the more interesting fields are:
| Name | Size (bits) |
Meaning |
|---|---|---|
| Version | 4 | The IP version number (6 in this case) |
| Priority | 4 | The priority is used to differentiate data like e-mail where speed is not of the essence from more demanding data like router-control information or audio or video data streams |
| Flow label | 24 | Identifier of information that will guide the routers in quickly passing packets that are of high priority |
| Source address | 128 | The IP address of the host sending the packet |
| Destination address | 128 | The IP address of the host the packet is being routed to |