In Datagram networks, also called Connectionless Networks, the communication between the two sites is on a one-off basis; the packet contains the full addressing information needed to transmit it. If a site sends two datagrams in quick succession to the same address they could arrive in reverse order for the intervening nodes are allowed to change the exact path through the network depending on traffic flows and to bypass faulty nodes. An analogy is the postal service; each packet is a separate letter.