The Data Link and Network layers of the OSI model are responsible for addressing local and network data. One layer uses logical addresses; the other layer uses physical addresses. This section briefly covers Media Access Control (MAC) addresses used at the Data Link layer and protocol addresses used at the Network layer. Although there are other Network-layer protocols, this section focuses on the implementation of Internet Protocol (IP) at this layer.

Critical Information

Let’s first concentrate on the Data Link layer, since this is the layer at which physical addresses that are assigned to network interface cards (NICs) are installed on the local hosts. Many people do not realize that there is actually a 48-bit address individually assigned to every NIC. Although you may buy a case of NICs from many different manufacturers, they all are coded with a unique MAC address.

Discussed later in this chapter, frames are data units at the Data Link layer (layer 2 of the OSI model). Each frame is composed of a Data Link Clayer header, data from the upper OSI layers, and a trailer. Cisco’s definition of what the Data Link layer provides is reliable transit of data across a physical network link. The OSI defines many specifications for this layer regarding different network and protocol characteristics. This includes the physical addressing, network topology, error notification, sequencing of frames, and flow control. Let’s take a look at each one:
Physical addresses are defined as the MAC addresses assigned to the NIC card at the Data Link layer.

• The network topology is how devices are connected to the network.
• The error notification process alerts the OSI model’s upper layers of a transmission error.
• Sequencing is important. If data frames arrive out of sequence, a real problem might occur if the receiving device had no way of knowing the correct sequence.
• Flow control is used to manage how many frames are sent to a receiving device to keep the receiver from being overwhelmed with more frames than it can process or buffer.

MAC addresses are divided into two parts: a 24-bit manufacturer’s identifier called the organizationally unique identifier (OUI), and a 24-bit vendor-supplied number or serial number that is unique to any other address the manufacturer has assigned to their cards. This makes MAC addresses 48 bits in length. These 48 bits are expressed with 12 hexadecimal digits, as in this example: 00D0.5966.A8AD or 00-D0- 59-66-A8-AD (depending on the operating system or software used to display it). The vendor code would be the 00-D0-59, and the serial number would be 66-A8-AD.

MAC addresses are called burned-in addresses (BIAs), or hardware addresses, because they are burned into read-only memory (ROM) on the installed host interface. The MAC address is copied into random access memory (RAM) when the interface initializes. The Network layer needs to map a logical address such as an IP address to the hardware address. Mappings can be statically created; however, mappings can be made dynamically using the Address Resolution Protocol (ARP).

The Network layer, also known as layer 3 of the OSI model, defines an address that differs significantly from the MAC address. Network- layer protocol addresses allow systematical comparison of the source network address and the destination network address. Routers use learned IP address information and routing protocols to make the best determination of how to route Network-layer data packets through the network. (Packets are the data unit used at the Network layer. They are composed of the Network-layer header, encapsulated upper-layer data, and a trailer.)

The CCNA exam focuses on two different types of logical addresses: IP addresses and IPX addresses. We will discuss these in more detail later in this chapter, but let’s look quickly at how IP and IPX addresses differ from the layer 2 MAC addresses.
An IP address is made up of 32 bits of information. These bits are divided into four sections, referred to as octets or bytes, each containing one byte (eight bits). Most often, IP addresses are shown in dotteddecimal form. An example would be 198.1.1.1.

An IPX address uses 80 bits, or 10 bytes, of data. The first four bytes show the network address, and the last six bytes always represent the node address, which is the MAC address. An example is 00007C80.0000.8609.33E9. The first eight hex digits (00007C80) represent the network portion of the address.

Related posts:

1. Data link and Network Addresses Exams Points
2. Five Conversion Steps of Data Encapsulation Points
3. TCP-IP Network-layer protocol Exam Points
4. IP Addresses and IPX Addresses
5. PPP Operations to Encapsulate WAN data on Cisco Routers
6. OSI Network layer Exam Points
7. Function of the MAC Address
8. Layered Model Exam Points
9. TCP-IP Network-layer protocol
10. Full and Half-Duplex Ethernet Operation