Certainly, here's the continuation and completion of the description of the IP datagram structure:

IP Datagram Structure:

• An IP datagram, the packet at the network layer under IP protocol, is highly structured and contains various fields.
• An IP datagram consists of two main sections: the header and the payload.

IP Datagram Header Fields:

1. Version (4 bits): Indicates the version of the Internet Protocol being used (IPv4 or IPv6).

2. Header Length (4 bits): Specifies the length of the header in 32-bit words. Almost always 20 bytes in length when dealing with IPv4.

3. Service Type (8 bits): Provides information about quality of service (QoS) and priorities.

4. Total Length (16 bits): Indicates the total length of the IP datagram.

5. Identification (16 bits): Used for grouping related datagrams, especially for fragmentation.

   

6. Flag (3 bits): Specifies fragmentation status (may be allowed to be fragmented, must not fragment, or already been fragmented, or more fragments).

7. Fragmentation Offset (13 bits): Used for reassembling fragmented datagrams.

8. Time to Live (TTL, 8 bits): Represents the maximum number of hops a datagram can traverse.

9. Protocol (8 bits): Indicates the transport layer protocol used (e.g., TCP, UDP).

10. Header Checksum (16 bits): Provides a checksum for the header contents.

11. Source IP Address (32 bits): Specifies the sender's IP address.

12. Destination IP Address (32 bits): Specifies the recipient's IP address.

13. IP Options (variable): Optional field for special characteristics, often for testing purposes.

14. Padding (variable): Used to ensure the header reaches the required size.