Networking
OSI Model

You can't take a network course without hearing about the OSI model. The OSI model describes how information moves from a program on a computer through the network. It's a model. When specific protocols (like TCP/IP) are created, the model is used to ensure operation with other networking components.

For the purposes of this course, you will only need to understand the first three layers.

1. Physical Layer - This is all the wire, connectors and hubs. Most networking problems in schools are here. Someone unplugs a computer's network wire from the wall. The electric is turned off to a classroom hub. A patch cord gets crimped, and shorts out. These are all physical layer problems.

The physical layer is the only layer with absolutely no intelligence. The components of the physical layer simply carry the message on the wire. That message can be in any language, with errors, encrypted... whatever. The physical layer moves the message along.

A hub is the most complex component of the physical layer. A hub (sometimes called a repeater) is the most basic connecting device of a star topology network. A hub is a "party line" communication device. Everything that goes into a hub is re-transmitted to all the devices connected to the hub.

2. Data Link Layer - This layer uses the MAC address (Media Access Control address) of Network Interface Cards (NICs) to determine the destination of a packet of information.

The MAC address is a unique hexadecimal serial number burned into every network interface card. Sometimes the MAC address is refered to as the "burned in address" or the "physical address". Each MAC address is 6 bytes in length and look something like this:

00-08-74-E9-44-39

The first three bytes specify the manufacturer. The last three bytes are specific to each NIC. What this means is that each manufacturer "programs" each NIC with a hard coded serial number.

When a device on the network sends a data packet, the first part of the packet contains the MAC address of the device that is to receive the packet. Only the NIC with the destination MAC address will read the packet. Many other NICs might see this packet as it goes across the network, but each will ignore this packet because it is not addressed for their MAC address. A "sniffer" will break this rule, but that is a different networking topic.

A switch is a Layer 2 device. This means that a switch will use the MAC address of the NIC connected to each port to determine where to send the traffic. NICs in other switch ports will not see the packets addressed to other NICs. This makes "sniffing" much more difficult on a network with switches.

3. Network Layer - The network layer uses the IP address in the packet to determine the destination. A router is a layer 3 device.

There is a device called a Layer 3 Switch. A Layer 3 Switch is not a normal switch. It has the ability to examine the IP address section of the packet, and "route" the packet to the switch that is connected to the NIC with the destination IP address. This can provide much faster packet delivery, but it will only be effective if all the switches in a network segment are Layer 3 Switches.