|  | 9.2 | Layer 3 Concepts
| 9.2.2 | Diagram the IP datagram
The Figure illustrates the format of an IP datagram. An IP datagram contains an IP header and data, and is surrounded by the Media Access Control (MAC) layer header and MAC layer trailer. One message may be transmitted as a series of datagrams that are reassembled into the message at the receiving location. The fields in this IP datagram are as follows: VERS -- version number HLEN -- header length, in 32-bit words type of service -- how the datagram should be handled total length -- total length (header + data) identification, flags, frag offset -- provides fragmentation of datagrams to allow differing MTUs in the internetwork TTL -- Time-To-Live protocol -- the upper-layer (Layer 4) protocol sending the datagram header checksum -- an integrity check on the header source IP address and destination IP address -- 32-bit IP addresses IP options -- network testing, debugging, security, and other options The protocol field determines the Layer 4 protocol being carried within an IP datagram. Although most IP traffic uses TCP, other protocols can also use IP. Each IP header must identify the destination Layer 4 protocol for the datagram. Transport layer protocols are numbered, similarly to port numbers. IP includes the protocol number in the protocol field.