Network+ Ch. 1

Ethernet: 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).

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. (Remember learning remainders from division back in elementary school?) The NIC sending the frame does a little math to make the CRC. Using binary arithmetic, it works a division problem on the data using a divisor called a

key. The result of this division is the CRC. When the frame gets to the receiving NIC, it divides the data by the same key. 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.

An IP address is known as a __ address to distinguish it from the physical address, the MAC address of the NIC.

logical

Large networks need a __ 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.

logical addressing

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.

For a TCP/IP network to send data successfully, the data must be wrapped up in two distinct containers. A frame of some type enables the data to

move from one device to another

Because most chunks of data are much larger than a single packet, they ...

must be chopped up before they can be sent across a network.

The receiving NIC strips away the Ethernet frame and passes the remaining packet off to the software. The __ handles all the rest of the work.

networking software built into your operating system

If you put an __ on the wire to measure voltage, you'd see something like Figure 1-14. An __ is a powerful tool that enables you to see electrical pulses.

oscilloscope

A __ is the PDU for Layer 3.

packet

For a TCP/IP network to send data successfully, the data must be wrapped up in two distinct containers. A frame of some type enables the data to move from one device to another. Inside that frame are both an IP-specific container that enables routers to determine where to send data—regardless of the physical connection type—and the data itself. In TCP/IP, that inner container is the

packet

At the Network layer, Layer 3, containers called __ get created and addressed so they can go from one network to another.

packets

IP uses a rather unique dotted decimal notation (sometimes referred to as a dotted-octet numbering system) based on four 8-bit numbers. Each 8-bit number ranges from 0 to 255, and the four numbers are separated by

periods

MAC addresses are also known as __ addresses.

physical

ecall 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.

What makes logical addressing powerful is another magic box—called a __—that connects each of the subnets,

router

Protocols are

sets of clearly defined rules, regulations, standards, and procedures that enable hardware and software developers to make devices and applications that function properly at a particular layer.

The IEEE forms MAC addresses from a numbering name space originally called , which simply means that the MAC address will be 48 bits, with the first 24 bits defining the OUI, just as described here.

"MAC-48"

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 (Figure 1-12). Note that ipconfig calls the MAC address the physical address, which is an important distinction, as you'll see a bit later in the chapter. (For macOS, type __ from a terminal;

"ifconfig"

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 (Figure 1-12). Note that ipconfig calls the MAC address the physical address, which is an important distinction, as you'll see a bit later in the chapter. (For macOS, type ifconfig from a terminal; for Linux, type "__" from a terminal to get similar results.)

"ip a"

Would you like to see the MAC address for your NIC? If you have a Windows system, type from a command prompt to display the MAC address

"ipconfig /all"

IP uses a rather unique dotted decimal notation (sometimes referred to as a dotted-octet numbering system) based on four 8-bit numbers. Each 8-bit number ranges from

0 to 255

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

Layer _ of the OSI model defines the method of moving data between computers

1

Like a canister, a frame can hold only a certain amount of data. Different types of networks use different sizes of frames, but the frames used in most wired networks hold at most __ bytes of data.

1500

All FCSs are only __ bytes long

4

Each hex character represents how many bits?

4

How many bits is the MAC address?

48

IP uses a rather unique dotted decimal notation (sometimes referred to as a dotted-octet numbering system) based on four __-bit numbers.

8

For a TCP/IP network to send data successfully, the data must be wrapped up in two distinct containers.

A frame of some type enables the data to move from one device to another. Inside that frame are both an IP-specific container that enables routers to determine where to send data—regardless of the physical connection type—and the data itself. In TCP/IP, that inner container is the packet.

Not all networks are Ethernet networks (although all wired networks at home or work always use Ethernet). Ethernet may dominate, but IP packets fit in all sorts of other connectivity options. Cable modems, for example, use a type of frame called

DOCSIS

Any device that deals with a MAC address is part of the __ Layer of the OSI model

Data Link (2)

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 will be 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.

This enables networks to connect across data lines that don't use

Ethernet, like the telephone network.

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 will be 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

The MAC address of __ is the Layer 2 broadcast address

FF-FF-FF-FF-FF-FF

Now that you've seen all the pieces used to send and receive frames, let's put these pieces together and see how a frame gets from one system to another. The basic send/receive process is as follows:

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. 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 1-23). 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.

The FCS uses a type of binary math called __ that the receiving NIC uses to verify that the data arrived intact.

a cyclic redundancy check (CRC)

Who issues the OUI to an organization?

IEEE

Routers use the__, not the MAC address, to forward data.

IP address

Using the MAC address is a great way to move data around, but this process raises an important question. How does a sending NIC know the MAC address of the NIC to which it's sending the data?

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. The MAC address of FF-FF-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.

The __ Protocol is the primary logical addressing protocol for TCP/IP.

Internet

IP stands for

Internet Protocol

What layer is the NIC typically part of?

Layer 2

The OSI seven layers are

Layer 7 Application Layer 6 Presentation Layer 5 Session Layer 4 Transport Layer 3 Network Layer 2 Data Link Layer 1 Physical

LLC stands for

Logical Link Control

Data Link layer sublayers

Logical Link Control (LLC), Media Access Control (MAC)

Switches, as you'll see in much more detail as we go deeper into networking, filter traffic by

MAC address

MAC stands for

Media Access Control

Cabling and hubs are in what layer?

Physical

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.

TCP stands for

Transmission Control Protocol

Like a canister, a frame can hold only a certain amount of data. Different types of networks use different sizes of frames, but the frames used in most wired networks hold at most 1500 bytes of data. This raises a new question: 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.

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 bit of checking information called the frame check sequence (FCS). The FCS uses a type of binary math called __ that the receiving NIC uses to verify that the data arrived intact.

a cyclic redundancy check (CRC)

To move past the physical MAC addresses and start using logical addressing requires some special software called

a network protocol

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.

The unit of data specified by a protocol at each layer of the OSI seven-layer model is called

a protocol data unit (PDU).

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 bit 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.

Any frame addressed specifically to another device's MAC address is called

a unicast frame.

For a TCP/IP network to send data successfully, the data must be wrapped up in two distinct containers. A frame of some type enables the data to move from one device to another. Inside that frame are both

an IP-specific container that enables routers to determine where to send data—regardless of the physical connection type—and the data itself.

The many steps a NIC performs to keep this data moving—sending and receiving frames over the wire, creating outgoing frames, reading incoming frames, and attaching MAC addresses—are classically broken down into two distinct jobs. 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.

The many steps a NIC performs to keep this data moving—sending and receiving frames over the wire, creating outgoing frames, reading incoming frames, and attaching MAC addresses—are classically broken down into two distinct jobs. The first job is the Logical Link Control (LLC). 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.

Getting data from one system to another in a simple network (defined as one in which all the computers connect to one switch) takes relatively little effort on the part of the NICs. But one problem with simple networks is that

computers need to broadcast to get MAC addresses. It works for small networks, but what happens when the network gets big, like the size of the entire Internet? Can you imagine millions of computers all broadcasting? No data could get through.

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.

What makes logical addressing powerful is another magic box—called a router—that

connects each of the subnets.

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.

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

Once you understand how data moves along the wire, the next question is, how does the network get the right data to the right system? All networks transmit data by breaking whatever is moving across the Physical layer (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.

The receiving NIC strips away the Ethernet frame and passes the remaining packet off to the software. The networking software built into your operating system handles all the rest of the work. The NIC's __ is the interconnection between the hardware and the software.

driver software

All networks transmit data by breaking whatever is moving across the Physical layer (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 __information and data for easier transmission. (More on this later in the chapter.)

encapsulates (puts a wrapper around

But IP packets don't leave their PC home without any clothes on! Each IP packet is handed to the NIC, which then

encloses the IP packet in a regular frame, creating, in essence, a packet within a frame.

A __ is the PDU for Layer 2.

frame

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

Networks use __ 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.

frames

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

MAC addresses are always written in (what form)?

hex

Once the packet reaches the destination subnet's router, that router will

strip off the incoming frame—no matter what type—look at the destination IP address, and then add a frame with the appropriate destination MAC address that matches the destination IP address.

When you send data from one computer to another on a TCP/IP network such as the Internet, that data can go through many routers before it reaches its destination. Each router

strips off the incoming frame, determines where to send the data according to the IP address in the packet, creates a new frame, and then sends the packet within a frame on its merry way.

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

So, what's inside the data part of the frame? You neither know nor care. 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.

The only OSI layer that has sublayers

the Data Link layer

The NIC is in what layer?

the Data Link layer and the Physical layer

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

Any company that makes NICs must contact __ and request a block of MAC addresses, which the company then burns into the ROMs on its NICs.

the Institute of Electrical and Electronics Engineers (IEEE)

In a TCP/IP network, each system has two unique identifiers:

the MAC address and the IP address.

Ethernet frame parts from first to last: 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

Networking professionals use one of two methods to conceptualize the many parts of a network:

the Open Systems Interconnection (OSI) seven-layer model and the Transmission Control Protocol/Internet Protocol (TCP/IP) model.

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)

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.

When you send data from one computer to another on a TCP/IP network such as the Internet, that data can go through many routers before it reaches its destination. Each router strips off the incoming frame, determines where to send the data according to the IP address in the packet, creates a new frame, and then sends the packet within a frame on its merry way. The new frame type will be

the appropriate technology for whatever connection technology connects to the next router. That could be a cable or DSL network connection, for example (Figure 1-32). The IP packet, on the other hand, remains unchanged.

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.

Ethernet frame parts from first to last:

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 bit 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.

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.

The receiving NIC strips away the Ethernet frame and passes the remaining packet off to the software. The networking software built into your operating system handles all the rest of the work. The NIC's driver software is the interconnection between the hardware and the software. The NIC driver knows how to communicate with the NIC to send and receive frames, but it can't do anything with

the packet. Instead, the NIC driver hands the packet off to other services that know how to deal with all the separate packets and turn them into Web pages, e-mail messages, files, and so forth.

Note that ipconfig calls the MAC address

the physical address

In a TCP/IP network, each system has two unique identifiers: the MAC address and the IP address. The MAC address (the physical address) is literally burned into the chips on the NIC, whereas the IP address (the logical address) is simply stored in

the system's software.

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

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.

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.

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.