What is a DATAGRAM in Networking...??

• Packets that goes through Datagram subnet are called datagrams. 

• Datagrams are independent packets having information about the sender, receiver & the message to be delivered.They take any route between the sender & receiver as there is no dedicated point-to-point channel established between them.

• Packets that goes through Virtual circuit subnet first establishes a channel and then all packets goes through that channel. These packets don’t have any information about the sender and the receiver.

Depending on the type of network, packets may be referred to by another name:

• In physical layer the actual data is in "bits" i.e. 0 and 1

• When it reaches to data link layer or Layer 2 it becomes "frame" with source and destination MAC address getting added

• When it reaches the third layer or the networking layer it becomes a "packet" with source and destination IP address attached to it

• Finally it become "segment" at Layer 4 or TCP/IP layer. Basically "segment" term is used for TCP connections and "datagrams" for UDP

• So what about other layers like : session, presentation and application ? In these layers the data is termed as "PDU" or "protocol data unit"