Fundamentals

• NOTE: The International Organization for Standardization (ISO) created the OSI seven-layer model. ISO may look like a misspelled acronym, but it’s actually a word, derived from the Greek word isos, which means “equal.” The International Organization for Standardization sets standards that promote equality among network designers and manufacturers, thus ISO.
• NICs:

  • On older systems, a NIC truly was a separate card that snapped into a handy expansion slot, which is why they were called network interface cards.
  • Even though they’re now built into the motherboard, they are still called NICs. Figure 1-9 shows a typical modern laptop with a dongle providing an Ethernet port. Note the cable runs from the NIC into the wall; inside that wall is another cable running all the way back to the central box.

  
• The NIC

  • The network must provide a mechanism that gives each system a unique identifier—like a telephone number—so data is delivered to the right system. That’s one of the NIC’s most important jobs. Inside every NIC, burned onto some type of ROM chip, is special firmware containing a unique identifier with a 48-bit value called the media access control address, or MAC address.
• The NIC

  • No two NICs ever share the same MAC address—ever. Any company that makes NICs must contact the Institute of Electrical and Electronics Engineers (IEEE) and request a block of MAC addresses, which the company then burns into the ROMs on its NICs.
• Mac Address:

  • Hexadecimal Aside A hexadecimal numbering system uses base 16 to represent numbers—that would be 0–15 (in base 10 values). Contrast this with the more common decimal numbering system, numbered 0–9. Just as with decimal, people who work with hexadecimal need a single character to represent each number for the 16 values. Using 0–9 makes sense, but then hex is represented in letter form for the values 10–15 (A, B, C, D, E, F).
• Back to MAC Addresses

  • The MAC address in Figure 1-11 is 004005-607D49, although in print, we represent the MAC address as 00–40–05–60–7D–49. The first six digits, in this example 00–40–05, represent the number of the NIC manufacturer. Once the IEEE issues those six hex digits to a manufacturer—referred to as the Organizationally Unique Identifier (OUI)—no other manufacturer may use them. The last six digits, in this example 60–7D–49, are the manufacturer’s unique serial number for that NIC; this portion of the MAC is often referred to as the device ID.
• Would you like to see the MAC address for your NIC?

  • If you have a Windows system, type ipconfig /all from a command prompt to display the MAC address ipconfig calls the MAC address the physical address.
  • For macOS, type ifconfig from a terminal; for Linux, type ip a from a terminal to get similar results.

  
• UTP cabling

  • Another key piece of hardware the network uses is a special box-like device that handles the flow of data from each computer to every other computer
  • This box is often tucked away in a closet or an equipment room. (The technology of the central box has changed over time. For now, let’s just call it the “central box.” I’ll get to variations in a bit.) Each system on the network has its own cable that runs to the central box.
• Typical central box

  • Layer 1 of the OSI model defines the method of moving data between computers, so the cabling and central box are part of the Physical layer
  • Anything that moves data from one system to another, such as copper cabling, fiber optics, even radio waves, is part of the OSI Physical layer. Layer 1 doesn’t care what data goes through; it just moves the data from one system to another system

  
• Dongle NIC with cable connecting the laptop to the wall jack Cabling and central boxes define the Physical layer of the network, and NICs provide the interface to the PC.
• The NIC

  • No two NICs ever share the same MAC address—ever. Any company that makes NICs must contact the Institute of Electrical and Electronics Engineers (IEEE) and request a block of MAC addresses, which the company then burns into the ROMs on its NICs. Many NIC makers also print the MAC address on the surface of each NIC, as shown in Figure 1-11. Note that the NIC shown here displays the MAC address in hexadecimal notation. Count the number of hex characters—because each hex character represents 4 bits, it takes 12 hex characters to represent 48 bits. MAC addresses are always written in hex.
• Hexadecimal works great with binary. Four bits provide the values of 0–15. 0001, for example, is the value 1; 1000 in binary is 8; 1111 is 15. When we work with MAC addresses, it’s far easier to break each 4-bit section of the 48-bit address and translate that into hex. Humans work better that way!
• MAC-48 and EUI-48

  • The IEEE forms MAC addresses from a numbering name space originally called MAC-48, which simply means that the MAC address is 48 bits, with the first 24 bits defining the OUI, just as described here. The current term for this numbering name space is EUI-48. EUI stands for Extended Unique Identifier. (IEEE apparently went with the latter term because they could trademark it.)
• MACs:

  • Recall that computer data is binary, which means it’s made up of streams of ones and zeroes. NICs send and receive this binary data as pulses of electricity, light, or radio waves. Let’s consider the NICs that use electricity to send and receive data.
  • The specific process by which a NIC uses electricity to send and receive data is exceedingly complicated but, luckily for you, not necessary to understand. Instead, just think of a charge on the wire as a one and no charge as a zero. A chunk of data moving in pulses across a wire might look something like

  
• Oscilloscope of data

  • Now, remembering that the pulses represent binary data, visualize instead a string of ones and zeroes moving across the wire

  
• Data as ones and zeroes

  • All networks transmit data by breaking whatever is moving across the Physical layer (such as files, print jobs, Web pages, and so forth) into discrete chunks called frames. A frame is basically a container for a chunk of data moving across a network. A frame encapsulates—puts a wrapper around— information and data for easier transmission. The NIC creates and sends, as well as receives and reads, these frames.
• Frames:

  • Different frame types are used in different networks. All NICs on the same network must use the same frame type, or they will not be able to communicate with other NICs.
  • This shows a representation of a generic frame, a simplified version of the wired network technology for home/office use, called Ethernet

  
• The unit of data specified by a protocol at each layer of the OSI seven-layer model is called a protocol data unit (PDU). A frame is the PDU for Layer 2
• Generic frame

  • Note that the frame begins with the MAC address of the NIC to which the data is to be sent, followed by the MAC address of the sending NIC. Next comes the Type field, which indicates what’s encapsulated in the frame. Then comes the Data field that contains what’s encapsulated, followed by a special piece of checking information called the frame check sequence (FCS). The FCS uses a type of binary math called a cyclic redundancy check (CRC) that the receiving NIC uses to verify that the data arrived intact.
• Generic Frame:

  • You can think of a frame in a different way as having three sections. The header (MAC addresses and Type) starts, followed by the payload (whatever is encapsulated in the frame); this is followed by the trailer (the FCS).
  • The data may be a part of a file, a piece of a print job, or part of a Web page. NICs aren’t concerned with content! The NIC simply takes whatever data is passed to it via its device driver and addresses it for the correct system. Special software will take care of what data gets sent and what happens to that data when it arrives

  
• Frames:

  • A frame can hold only a certain amount of data. Different types of networks use different sizes of frames, but the frames used in Ethernet networks hold at most 1500 bytes of data
  • What happens when the data to be sent is larger than the frame size? Well, the sending system’s software must chop the data up into nice, frame-sized chunks, which it then hands to the NIC for sending. As the receiving system begins to accept the incoming frames, the receiving system’s software recombines the data chunks as they come in from the network.
• Into the Central Box

  • When a system sends a frame out on the network, the frame goes into the central box. What happens next depends on the technology of the central box.
  • In the early days of networking, the central box was called a hub. A hub was a dumb device, essentially just a repeater. When it received a frame, the hub made an exact copy of that frame, sending a copy of the original frame out of all connected ports except the port on which the message originated.
• Hub:

  • Only the NIC to which the frame was addressed would process that frame—the other NICs simply dropped it when they saw that it was not addressed to their MAC address. This is important to appreciate: with a hub, every frame sent on a network was received by every NIC, but only the NIC with the matching MAC address would process that frame

  
• Later networks replaced the hub with a smarter device called a switch. Switches, as you’ll see in much more detail as we go deeper into networking, filter traffic by MAC address. Rather than sending all incoming frames to all network devices connected to it, a switch sends the frame only to the interface associated with the destination MAC address.
• FCS in Depth

  • All FCSs are only 4 bytes long, yet the wired frame carries at most 1500 bytes of data. How can 4 bytes tell you if all 1500 bytes in the data are correct? That’s the magic of the math of the CRC. Without going into the grinding details, think of the CRC as just the remainder of a division problem. The NIC sending the frame does a little math to make the CRC. The receiving NIC applies the same math. If the receiving NIC’s answer is the same as the CRC, it knows the data is good; if it’s not good, the frame is dropped.
• Getting the Data on the Line

  • Because all the data sent by one NIC is read by every other NIC on the network, only one system could speak at a time in early wired networks. Networks use frames to restrict the amount of data a NIC can send at once, giving all NICs a chance to send data over the network in a reasonable span of time.
• MAC:

  • In most cases, the sending system already knows the destination MAC address because the NICs had probably communicated earlier, and each system stores that data. If it doesn’t already know the MAC address, a NIC may send a broadcast onto the network to ask for it.
• MAC:

  • The MAC address of FFFF-FF-FF-FF-FF is the Layer 2 broadcast address—if a NIC sends a frame using the broadcast address, every single NIC on the network will process that frame. That broadcast frame’s data will contain a request for a system’s MAC address. Without knowing the MAC address to begin with, the requesting computer will use an IP address to pick the target computer out of the crowd
  • The system with the MAC address your system is seeking will read the request in the broadcast frame and respond with its MAC address
• Frame Movement:

  • First, the sending system’s operating system hands some data to its NIC. The NIC builds a frame to transport that data to the receiving NIC
  • After the NIC creates the frame, it adds the FCS, and then dumps it and the data into the frame
  • Next, the NIC puts both the destination MAC address and its own MAC address onto the frame. It then sends the frame through the cable to the network
• NOTE: Any frame addressed specifically to another device’s MAC address is called a unicast frame. The one-to-one addressing scheme is called unicast addressing; you’ll see it in other layers as well as Layer 2.

  
• The frame propagates down the wire into the central box. The switch sends unicast frames to the destination address and sends broadcast frames to every system on the network. The NIC receives the frame (Figure 1-24). The NIC strips off all the framing information and sends the data to the software—the operating system—for processing. The receiving NIC doesn’t care what the software does with the data; its job stops the moment it passes on the data to the software.
• NICs:

  • Any device that deals with a MAC address is part of the OSI Data Link layer, or Layer 2 of the OSI model. Let’s update the OSI model to include details about the Data Link layer
  • Note that the cabling (and hubs) are in the Physical layer. Switches handle traffic using MAC addresses, so they operate at Layer 2. That’s the way modern wired networks work. The NIC is in the Data Link layer and the Physical layer.

  
• NICs:

  • The first job is called the Logical Link Control (LLC). The LLC is the aspect of the NIC that talks to the system’s operating system (usually via device drivers). The LLC handles multiple network protocols and provides flow control.
• NICs:

  • The second job is called the Media Access Control (MAC), which creates and addresses the frame. It adds the NIC’s own MAC address and attaches MAC addresses to the frames. Recall that each frame the NIC creates must include both the sender’s and recipient’s MAC addresses. The MAC sublayer adds or checks the FCS. The MAC also ensures that the frames, now complete with their MAC addresses, are then sent along the network cabling.
• Large networks need a logical addressing method, like a postal code or telephone numbering scheme, that ignores the hardware and enables you to break up the entire large network into smaller networks called subnets. Figure 1-27 shows two ways to set up a network. On the left, all the computers connect to a single switch. On the right, however, the LAN is separated into two five-computer subnets.

  
• Network Protocols:

  • To move past the physical MAC addresses and start using logical addressing requires some special software called a network protocol. Network protocols exist in every operating system. A network protocol not only has to create unique identifiers for each system, but also must create a set of communication rules for issues like how to handle data chopped up into multiple packets and how to ensure those packets get from one subnet to another
• Network Layer:

  • At the Network layer, Layer 3, containers called packets get created and addressed so they can go from one network to another. The Internet Protocol is the primary logical addressing protocol for TCP/IP. IP makes sure that a piece of data gets to where it needs to go on the network. It does this by giving each device on the network a unique numeric identifier called an IP address. An IP address is known as a logical address to distinguish it from the physical address, the MAC address of the NIC.
• IPs:

  • IP uses a rather unique dotted decimal notation based on four 8- bit numbers. Each 8-bit number ranges from 0 to 255, and the four numbers are separated by three periods. 
• No two systems on the same network share the same IP address; if two machines accidentally receive the same address, unintended side effects may occur. These IP addresses don’t just magically appear—they must be configured by the network administrator.
• Router

  • Connects each of the subnets, as previously shown in Figure 1-27. Routers use the IP address, not the MAC address, to forward data. This enables networks to connect across data lines that don’t use Ethernet, like the telephone network. Each network type (such as Ethernet, SONET, and others that we’ll discuss later in the book) uses a unique frame.