Is network packet synonym to IP datagram?

Question

The word packet as in packet-switched network is often mentioned in context of various layers, from transport to link and I am slightly confused and I do not know what layer it refers to (if there is any particular layer).

From what I know it seems to many that packet = IP datagram but I do not understand why is the word IP datagram used instead (and also used for transport layer UDP PDU) and why is the word packet used when discussing TCP or link layer. Is that correct? Is it probably synonym for PDU?

Answer 1

As TCP/IP networks dominate the most networks, network packet = IP datagram is true to some extent. However following exceptions do exist:

1. One IP datagram may be split into multiple packets if the physical network can only transport smaller packets.

2. Ethernet packets do not necessarily transport IP datagrams, there are other protocols in use, examples are ARP and AppleTalk.