What data is transmitted at the network layer?

At the network layer (L3), packets are transmitted - this is the basic data unit of this layer.

Short and precise: the network layer works with packets, usually IP packets.

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#What exactly is transmitted at the network layer

#1. Packets (IP packets)

The network layer:

• receives data from the transport layer (TCP/UDP)
• packs it into a packet
• transmits the packet through one or more networks

A packet is:

• data + service information of the network layer

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#2. IP header

Every packet contains an IP header, which includes:

• the source IP address
• the destination IP address
• routing information
• service fields (TTL, protocol type, etc.)

It is this header that allows:

• understanding where to direct the packet
• making decisions at routers

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#3. Payload

Inside an IP packet is:

• a transport layer segment (TCP or UDP)

The network layer:

• does not analyze the content of the payload
• simply delivers the packet further

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#What is NOT transmitted at the network layer

Important for an interview The network layer does not work with:

• bits as signals (that is L1)
• frames and MAC addresses (that is L2)
• application ports (that is L4)
• application data directly

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#Relationship between data units across layers

• Physical layer → bits
• Data link layer → frames
• Network layer → packets
• Transport layer → segments

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#Where packets exist

• packets are transmitted between networks
• routers work specifically with packets
• at every hop between networks:
  • the IP packet is preserved
  • the L2 frame is rebuilt from scratch