Lesson 2: OSI Reference Model

The Layered Model | Physical & Data Link Layers | Network Layer | Transport, Session, Presentation, and Application Layers

Layers 1 & 2: Physical & Data Link Layers

Now let’s take a look at each of the layers in a bit more detail and with some context. For Layers 1 and 2, we’re going to look at physical device addressing, and the resolution of such addresses when they are unknown.

Physical and Logical Addressing

Locating computer systems on an internetwork is an essential component of any network system – the key to this is addressing.
Every NIC card on the network has its own MAC address. In this example we have a computer with the MAC address 000.0C12.3456. The MAC address is a hexadecimal number so the numbers in this address here don’t go just from zero to nine, but go from zero to nine and then start at "A" and go through "F". So, there are actually sixteen digits represented in this counting system. Every type of device on a network has a MAC address, whether it is a Macintosh computer, a Sun Work Station, a hub or even a router. These are known as physical addresses and they don’t change.
Logical addresses exist at Layer 3 of the OSI reference model. Unlike link-layer addresses, which usually exist within a flat address space, network-layer addresses are usually hierarchical. In other words, they are like mail addresses, which describe a person’s location by providing a country, a state, a zip code, a city, a street, and address on the street, and finally, a name. One good example of a flat address space is the U.S. social security numbering system, where each person has a single, unique security number.

MAC Address

For multiple stations to share the same medium and still uniquely identify each other, the MAC sub layer defines a hardware or data link address called the MAC address. The MAC address is unique for each LAN interface.
On most LAN-interface cards, the MAC address is burned into ROM—hence the term, burned-in address (BIA). When the network interface card initializes, this address is copied into RAM.
The MAC address is a 48-bit address expressed as 12 hexadecimal digits. The first 6 hexadecimal digits of a MAC address contain a manufacturer identification (vendor code) also known as the organizationally unique identifier (OUI). To ensure vendor uniqueness the Institute of Electrical and Electronic Engineers (IEEE) administers OUIs. The last 6 hexadecimal digits are administered by each vendor and often represent the interface serial number.

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