Networking - The TCP/IP Five-Layer Model - The Network Layer

The Network layer in the TCP/IP model is often called the Internet layer. It is responsible for moving packets from one network to another so that they eventually reach the correct destination host.

In particular, the Network layer is responsible for:

• Providing logical addressing so that each host can be identified throughout an internetwork, and
• Selecting paths through interconnected networks and forwarding packets along those paths.

The primary protocol at this layer is the Internet Protocol (IP). Supporting protocols such as ICMP, ARP, and routing protocols also belong conceptually to the Network layer.

The Network layer is responsible for several processes, each of which is responsible for a series of methods:

1. Logical Addressing
   • Assigning IP addresses to interfaces so that hosts and routers can be uniquely identified, and
   • Dividing address space into networks and subnets using masks or prefix lengths.
2. Routing and Forwarding
   • Building routing tables using static routes or dynamic routing protocols, and
   • Forwarding packets based on their destination addresses and the routing table.
3. Fragmentation and Reassembly
   • Breaking large packets into fragments when they must cross links with smaller maximum transmission units, and
   • Reassembling those fragments at the destination Network layer.

Devices most closely associated with the Network layer include routers and Layer 3 switches, which make forwarding decisions based on IP addresses rather than on Data Link layer addresses.

Let us move on to layer 4, the Transport Layer.

Nah, I want to skip around:

• Introduction
• Physical (Layer 1)
• Data Link (Layer 2)
• Network (Layer 3)
• Transport (Layer 4)
• Application (Layer 5)
• Conclusion

Copyright 1999, Marc Elliot Hall, DBA Sensation! Services