Routing and Networking Tutorial (AKA CCNA Prep)
Hello everyone!
I am currently half way through taking the four exams required for me to get my Cisco Certified Networking Professional certification, so I figured I'd write a quick tutorial on some of the networking basics required to get the Cisco Certified Networking Associate certification (minus the router commands portion), and even if you aren't looking for the certification it is helpful to get the basic ideas down, cause they certainly help in network security situations.
First, we'll start with the Open Systems Interconnection (OSI) model. In the OSI model, there are 7 layers; they are as follows:
Application Layer: Handles communications between applications and the lower OSI layers. (FTP, SMTP, Telnet)
Presentation Layer: Handles encoding of data (JPEG, GIF, MPEG)
Session Layer: Manages network session information. (Remote Procedure Calls, etc..)
Transport Layer: Handles reliability issues, flow control, and other connection functions (TCP, SPX)
Network Layer: Handles packet routing (Open Shortest Path First router, Border Gateway Protocol routing etc..)
Data Link Layer: The Data Link Layer is split into two parts(PPP, Frame Relay, etc..)
1. Media Access Control: Manages physical access to the network
2. Logical Link Control: Manages communications between devices
Physical: The physical medium (Ethernet, FDDI, Token Ring)
Now that we have an idea of the layers used in communicating through computer networks, we can now move into how information changes from application information into packets and finally into bits that are physically sent across the network.
User data from an application traverses the OSI layers, and when it reaches the Transport layer, it is changed to what is called a Segment where a TCP header is added to the data. After that, it is moved to the Network layer where the Segment is changed to a Packet and an IP header is added to the previous data. The data now moves to the Data Link layer, and it is now turned into a Frame and the Logical Link Control header is added. Finally, the data moves to the Physical layer and a Media Access Control header is added and the data is turned into Bits which are then physically sent on the network wire.
One of the most important reasons for learning the OSI model layers is to know in which layers networking devices, such as routers or firewalls, work.
HUBs, for instance, work on layer 1, or the physical layer of the OSI model. Switches and bridges work on the data link layer (layer 2), and finally routers work on the network layer (layer 3).
For HUBs, they merely repeat the bits that they receive, therefore they are layer 1 devices. Switched and bridges use MAC addresses, therefore they are layer 2 devices. Finally, routers use network addresses to perform their routing operations, therefore they are layer 3 devices.
I hope you enjoyed the tutorial thus far. I'm not sure exactly what the interest is in the lower level (i.e. not just Linksys gateways) routing and switching info, so you guys (and gals) should let me know if you want me to go into more detail on routing and switching protocols. Maybe classful and classless subnetting. Just let me know and I'll throw a part 2 together.
Regards,
Wizeman
