What is the PDU at Layer 3 (Network)?

The Layer 3 (Network) PDU is the packet — a segment wrapped with an IP header carrying source/destination IP addresses for routing.

* The Layer 3 packet: IP header plus the transport segment. *

The Network layer takes a segment from the Transport layer and wraps it in an IP header to make a packet (also called a datagram for IP). The whole reason for this layer is to make data routable across different networks, so the header it adds is all about global addressing and forwarding:

• Source and destination IP addresses — the end-to-end "from / to" that stays constant across every router hop.
• TTL (Time to Live) — a hop counter that's decremented at each router and drops the packet at zero, so a misrouted packet can't loop forever.
• Protocol field — tells the receiving Network layer what's inside (e.g. TCP vs UDP) so it knows which Transport protocol to hand the data to.

A router reads only the packet's destination IP to pick the next hop — it doesn't care what segment or data is inside.

Memory aid: A packet is like a postal package — it carries the addresses routers use to forward it across the whole journey.

Go deeper:

• Network layer — the layer that builds packets and forwards them between networks.
• IP address — the source/destination addresses carried in the packet header.