What does the OSI network layer do?

The network layer (Network Layer) is responsible for delivering packets between different networks and for routing: choosing the path the data will take.

Short formulation for an interview:

The network layer provides logical addressing (IP addresses) and routes packets between networks. It is the layer that decides where to send a packet next - which node becomes the next "hop".

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#Main functions of the network layer

#1. Routing: the main function

The network layer:

• decides "where to send the packet"
• chooses the optimal path (route)
• passes the packet to the next router (next-hop)

This is implemented using:

• routing tables
• route lookup algorithms
• static/dynamic routes

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#2. Logical addressing (IP addressing)

Layer 3 uses IP addresses, which:

• are unique within the network
• are independent of MAC addresses
• allow packets to be delivered across multiple networks

Main addressing protocols:

• IPv4
• IPv6

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#3. Packet encapsulation and decapsulation

On the way in, the network layer:

• receives a segment from L4 (TCP/UDP)
• wraps it into a packet (IP header + payload)

On the way out:

• passes the packet to the data link layer (L2)

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#4. Packet fragmentation and reassembly

If a packet exceeds the link's MTU, the network layer can:

• fragment it into smaller pieces (in IPv4)
• reassemble the fragments at the destination device

IPv6 does not support fragmentation by routers, only by the sender.

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#5. Determining packet lifetime (TTL/Hop Limit)

Every IP packet contains:

• TTL (IPv4)
• Hop Limit (IPv6)

Every router decrements the TTL by 1. If TTL=0, the packet is dropped and an ICMP "Time Exceeded" message is sent.

This protects the internet from infinite loops.

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#6. Handling ICMP messages

ICMP works on top of IP and is used for:

• transmission errors
• diagnostics
• ping and traceroute

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#Routers operate at the network layer

Switch = L2 Router = L3

A router:

• reads the destination IP address
• checks the routing table
• forwards the packet to the correct subnet

Without the network layer, it is impossible to deliver packets:

• to other subnets
• to the internet
• between offices/branches

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#Network layer protocols

#Addressing protocols:

• IPv4
• IPv6

#Diagnostic and service protocols:

• ICMP
• ICMPv6

#Routing protocols:

• OSPF
• BGP
• RIP
• IS-IS

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#What the network layer does NOT do

It's important not to confuse these:

The network layer is not responsible for:

• delivering frames within a LAN
• MAC addresses
• TCP/UDP ports
• reliable delivery

That is the job of L2 and L4.

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#Example of the network layer at work

If a computer with the address 192.168.1.5 sends a packet to 8.8.8.8:

1. L4 creates a segment (TCP/UDP).
2. L3 wraps it into an IP packet and checks the destination IP.
3. It realizes the address is on a different network.
4. It sends the packet to the router (gateway).
5. The router determines the best path and forwards it further.