Time has come to take one more step towards our goal of mastering a network and talk about the second layer of the OSI reference model, the Data Link Layer. If you feel like you want to go through the OSI layered model once more time, try reading the first article of this series, Master A Network - Part I.

The Data Link Layer deals with problems such as hardware addresses, network topologies and how computers on a network access the shared medium.

Before talking about Layer 2 protocols (such as Ethernet and Token Ring) I'd like to briefly go over the process of encapsulation one more time. As data from the Application layer travels down the Layer stack, it is broken down into smaller units and each unit receives additional information (headers and trailers). These units have particular names, one for each layer that encapsulates data. The Layer 2 encapsulated data is called a frame. The extra bytes in the header and trailer identify the source and the destination of transmitted data and also help in finding erroneous packets, thus ensuring reliable network communications. This process is very similar to mailing a letter to a friend. You place the letter (data from upper layers) into an envelope and on the envelope you write the friend's address (the destination address). The information on the envelope ensures that your letter is properly directed by the postal service (the devices on the network) until it finally reaches your friend's mailbox.

Every computer connects to a network through a Network Interface Card (NIC). The NIC is a printed circuit board that translates the user data into a form of energy that can travel on the network media (electrical impulses, pulses of light, electromagnetic waves). Each NIC has a unique address burnt into a ROM chip on the board. This address is called the MAC address (Media Access Control) and it's 48-bits long (12 hex digits). The MAC addresses consist of two (equal) parts. The first six hex digits (referred to as the OUI - Organizational Unique Identifier) identify the manufacturer of the NIC. The manufacturer randomly assigns the later six. Given the fact that there are 12 hex digits in each MAC address there are 12^16 (= 184,884,258,895,036,416) possible combinations, so the MAC addresses won't run out any time soon ;). There are two formats used to specify a MAC address: six groups of two hex digits (e.g. 00-d0-09-31-b4-94) or three groups of four hex digits (e.g. 0004-9a87-1ec8). Later in this article you will see how the computers use this MAC address to communicate with each other.

I've stated earlier that the Data Link Layer is where the network topology is defined. There are actually two aspects of a network's topology: the physical topology (the actual layout of the wires) and the logical topology (how data flows in a network). There are six basic topologies: bus (linear), ring, star, extended star, hierarchical and mesh.