The Network Layer is the third layer in the OSI (Open Systems Interconnection) model. It is responsible for the delivery of packets from the source host to the destination host across multiple networks. This layer handles the routing of data, ensuring that it reaches its intended destination efficiently and accurately.

1. Routing: Determining the optimal path for data to travel from source to destination.
2. Logical Addressing: Assigning unique identifiers (IP addresses) to devices on the network.
3. Packet Forwarding: Moving packets from one network segment to another.
4. Fragmentation and Reassembly: Breaking down large packets into smaller ones for transmission and reassembling them at the destination.

Logical addressing is a crucial function of the Network Layer. Each device on a network is assigned a unique IP address, which helps in identifying the source and destination of data packets.

An IPv4 address is a 32-bit number, typically represented in decimal format as four octets separated by periods (e.g., 192.168.1.1).

An IPv6 address is a 128-bit number, represented in hexadecimal format and separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).

Routing is the process of selecting paths in a network along which to send network traffic. Routers are devices that perform this function by using routing tables and protocols.

A routing table is a data table stored in a router or a networked computer that lists the routes to particular network destinations.

Routing protocols are used to facilitate the exchange of routing information between routers. Some common routing protocols include:

• RIP (Routing Information Protocol): A distance-vector routing protocol that uses hop count as a routing metric.
• OSPF (Open Shortest Path First): A link-state routing protocol that uses the shortest path first algorithm.
• BGP (Border Gateway Protocol): A path-vector protocol used for routing between autonomous systems on the internet.

Packet forwarding is the process of receiving a packet on one interface of a router and sending it out on another interface towards its destination.

1. Receive Packet: The router receives a packet on one of its interfaces.
2. Check Routing Table: The router checks its routing table to determine the best path to the destination.
3. Forward Packet: The router forwards the packet to the next hop or the destination interface.

When a packet is too large to be transmitted over a network, it is fragmented into smaller packets. These fragments are reassembled at the destination.

Consider a packet of 4000 bytes that needs to be sent over a network with an MTU (Maximum Transmission Unit) of 1500 bytes.

• Original Packet: 4000 bytes
• Fragment 1: 1500 bytes
• Fragment 2: 1500 bytes
• Fragment 3: 1000 bytes

Given the following routing table, determine the next hop for a packet destined for 172.16.0.10.

Solution: The destination network for 172.16.0.10 is 172.16.0.0 with a subnet mask of 255.255.255.0. According to the routing table, the next hop is 172.16.0.1 via interface eth0.

In this section, we covered the Network Layer of the OSI model, focusing on its key functions such as routing, logical addressing, packet forwarding, and fragmentation and reassembly. We also explored examples of IPv4 and IPv6 addressing, routing tables, and routing protocols. Finally, we provided a practical exercise to reinforce the concepts learned.

Next, we will delve into the Transport Layer, which is responsible for end-to-end communication and error handling in the OSI model.