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Network Basics: Network Layers

Computers on a network communicate in agreed upon ways called protocols. The complexity of networking protocol software calls for the problem to be divided into smaller pieces. A layering model aids this division and provides the conceptual basis for understanding how software protocols together with hardware devices provide a powerful communication system.


Layering Models

In the early days of networking, before the rise of the ubiquitous Internet, the International Organization for Standardization (ISO) developed a layering model whose terminology persists today. The 7-layer model has been revised to the 5-layer TCP/IP reference model to meet the current needs of protocol designers.

ISO 7-Layer Reference Model:
  • Layer 7: Application - Specifies how a particular application uses a network
  • Layer 6: Presentation - Specifies how to represent data
  • Layer 5: Session - Specifies how to establish communication with a remote system
  • Layer 4: Transport - Specifies how to reliably handle data transfer
  • Layer 3: Network - Specifies addressing assignments and how packets are forwarded
  • Layer 2: Data Link - Specifies the organization of data into frames and how to send frames over a network
  • Layer 1: Physical - Specifies the basic network hardware


TCP/IP 5-Layer Reference Model:
  • Layer 5 Application - Specifies how a particular application uses a network
  • Layer 4 Transport - Specifies how to ensure reliable transport of data
  • Layer 3 Internet - Specifies packet format and routing
  • Layer 2 Network - Specifies frame organization and transmittal
  • Layer 1 Physical - Specifies the basic network hardware


TCP/IP Protocol Stack

TCP/IP is the protocol suite upon which all Internet communication is based. Different vendors have developed other networking protocols, but even most network operating systems with their own protocols, such as Netware, support TCP/IP. It has become the de facto standard. Protocols are sometimes referred to as protocol stacks or protocol suites. A protocol stack is an appropriate term because it indicates the layered approach used to design the networking software.



Each host or router in the internet must run a protocol stack. The details of the underlying physical connections are hidden by the software. The sending software at each layer communicates with the corresponding layer at the receiving side through information stored in headers. Each layer adds its header to the front of the message from the next higher layer. The header is removed by the corresponding layer on the receiving side.
This chapter discusses the protocols available in the TCP/IP protocol suite. The following figure shows how they correspond to the 5-layer TCP/IP Reference Model. This is not a perfect one-to-one correspondence; for instance, Internet Protocol (IP) uses the Address Resolution Protocol (ARP), but is shown here at the same layer in the stack.