Final answer:
The ninth octet in the header of an IP datagram is significant because it contains the Time to Live (TTL) field. The TTL field limits the lifespan of the packet and prevents it from circulating indefinitely in the network.
Step-by-step explanation:
In the header of an IP datagram, also known as an IP packet, each octet has a specific significance. The ninth octet is part of the IPv4 header and is reserved for a field called the Time to Live (TTL). The TTL field is used to limit the lifespan of the packet and prevent it from circulating indefinitely in the network.
Whenever a router receives an IP datagram, it decrements the TTL field value by one. If the TTL reaches zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message back to the source. This mechanism helps prevent routing loops and ensures that packets do not congest the network.
For example, if a packet has a TTL value of 10 and it goes through 10 routers before reaching its destination, each router will decrement the TTL value by one. When the packet reaches the 11th router, the TTL will become zero, and the router will drop the packet.