In 1999 I wrote a series of Border Gateway Protocol tutorials including a tutorial on autonomous system numbers. Since then AS numbers have been changed from 16-bit to 32-bit numbers to avoid running out of identifiers for BGP sessions. I have updated theAS numbers tutorial with a table that outlines what each range of autonomous system numbers are used for.

We covered the OSI Model in CCNA Lesson 5, now we move on to the TCP/IP Model which you will also need to know for the CCNA exam.   Since internetworking is based on the TCP/IP protocol suite, the TCP/IP model is a bit more important than the OSI model on the CCNA exam.

While the ISO folks were meeting in committees to develop the OSI Reference Model, the rest of the world got busy making networks actually work and the TCP/IP suite of protocols is the result.  The TCP/IP model of networking describes how the TCP/IP protocol suite functions and operates. Simpler than the OSI Reference Model, the TCP/IP model describes the most common stack of network protocols in use today.  This network model goes by several names:   the DARPA Model, the Department of Defense (DoD) Model,  and today we just call it the TCP/IP Model.


  • Four Layers
  • De-facto standard networking
  • Models the TCP/IP protocol suite

There is a full tutorial on the TCP/IP Model here at

There are two basic network models, the OSI Model and the TCP/IP Model.  Both outline the basic functions of how networks work and are each a standard in their own right. Understanding the OSI Reference Model is a requirement for passing any exam on networking whether it is the CCNA, CCNP, Network+ or any Juniper exam.

Memorize the OSI Reference Model. Now. You’ll use it over and over again and none of the information we cover from here on will make any sense if you don’t.  Networks were built one layer at a time over a period of time.  First physical communication was made possible, then logical addresses were used to  allow you to move computers around on a network. Next, the ability to route packets from one network to another was worked out, then ways to guarantee delivery of large volumes of data and finally ways to boot up and to manage the network, provide names humans can remember, and finally to serve up stored data as web pages.

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Networks can be described in terms of their physical layout, called a topology. A network's topology will often determine the characteristics of the network, whether it broadcasts data or communicates point-to-point, whether it will have unidirectional or bidirectional communication, how many end stations can be attached to it and many other factors. The logical topology will determine which end stations attached to it operate in discrete units and how data will be exchanged. Continue reading