One very interested bit I in the packet IP hearder is the DF or Don't Fragment. If the packet is set to 1, the routes accross the network wil not fragment the packet.
As an application example let's suppose we try to ping one router from another one.
First of all, we verify that the destination router is reachable:
Next, we are going to send different packets size with the DF enabled. This could help us to figure out which is the lower MTU packet size the the networks allow:
As we can see the Mtu configured is 1500 (the default value for a cisco router). In case we didn't enable the DF when sending a packet bigger than 1500 the router would fragment it in smaller packets:
