The TCP/IP model both defines and references a large collection of protocols that allow computers to communicate. To define a protocol, TCP/IP uses documents called Requests for Comments (RFC). (You can find these RFCs using any online search engine.) The TCP/IP model also avoids repeating work already done by some other standards body or vendor consortium by simply referring to standards or protocols created by those groups.
For example, the Institute of Electrical and Electronic Engineers (IEEE) defines Ethernet LANs; the TCP/IP model does not define Ethernet in RFCs, but refers to IEEE Ethernet as an option. An easy comparison can be made between telephones and computers that use TCP/IP. You go to the store and buy a phone from one of a dozen different vendors. When you get home and plug in the phone to the same cable in which your old phone was connected, the new phone works. The phone vendors know the standards for phones in their country and build their phones to match those standards.
Similarly, when you buy a new computer today, it implements the TCP/IP model to the point that you can usually take the computer out of the box, plug in all the right cables, turn it on, and it connects to the network. You can use a web browser to connect to your favorite website. How?
Well, the OS on the computer implements parts of the TCP/IP model. The Ethernet card, or wireless LAN card, built into the computer implements some LAN standards referenced by the TCP/IP model. In short, the vendors that created the hardware and software implemented TCP/IP.
To help people understand a networking model, each model breaks the functions into a small number of categories called layers. Each layer includes protocols and standards that relate to that category of functions. TCP/IP actually has two alternative models, as shown in Figure 1.9.
Figure 1.9 The Two TCP/IP Networking Models
The model on the left, the original TCP/IP model, breaks TCP/IP into four layers. The top layers focus more on the applications that need to send and receive data, whereas the lower layers focus more on the need to somehow transmit the bits from one device to another. The model on the right is a newer version of the model, formed by expanding the network access layer on the left into two separate layers: data link and physical. Note that the model on the right is used more often today.
Many of you will have already heard of several TCP/IP protocols, like the examples listed in
Table 1.3 TCP/IP Architectural Model and Example Protocols
TCP/IP Application Layer
TCP/IP application layer protocols provide services to the application software running on a computer. The application layer does not define the application itself, but it defines services that applications need. For example, application protocol HTTP defines how web browsers can pull the contents of a web page from a web server. In short, the application layer provides an interface between software running on a computer and the network itself.
Table 1.3 TCP/IP Architectural Model and Example Protocols Arguably, the most popular TCP/IP application today is the web browser. Many major software vendors either have already changed or are changing their application software to support access from a web browser.
OSI Layers and Their Functions
Cisco requires that CCNAs demonstrate a basic understanding of the functions defined by each OSI layer, as well as remembering the names of the layers. You understand which layers of the OSI model most closely match the functions defined by that device or protocol.
Today, because most people happen to be much more familiar with TCP/IP functions than with OSI functions, one of the best ways to learn about the function of different OSI layers is to think about the functions in the TCP/IP model, and correlate those with the OSI model.
If you use the five-layer TCP/IP model, the bottom four layers of OSI and TCP/IP map closely together. The only difference in these bottom four layers is the name of OSI Layer 3 (network)
compared to TCP/IP (Internet). The upper three layers of the OSI reference model (application, presentation, and session—Layers 7, 6, and 5) define functions that all map to the TCP/IP application layer. Table 1.4 defines the functions of the seven layers.
Table 1.4 OSI Reference Model Layer Definitions
Table 1.5 lists most of the devices and protocols covered in the CCNA exams and their comparable OSI layers. Note that many network devices must actually understand the protocols at multiple OSI layers, so the layer listed in Table 1.5 actually refers to the highest layer that the device normally thinks about when performing its core work. For example, routers need to think about Layer 3 concepts, but they must also support features at both Layers 1 and 2.
Besides remembering the basics of the features of each OSI layer (as in Table 1.4), and some example protocols and devices at each layer (as in Table 1.5), you should also Layer Functional Description 4 Layer 4 protocols provide a large number of services, “Fundamentals of TCP/IP Transport, Applications, and Security.” Although OSI Layers 5 through 7 focus on issues related to the application, Layer 4 focuses on issues related to data delivery to another computer (for instance, error recovery and flow control).
3 The network layer defines three main features: logical addressing, routing (forwarding), and path determination. Routing defines how devices (typically routers) forward packets to their final destination. Logical addressing defines how each device can have an address that can be used by the routing process. Path determination refers to the work done by routing protocols to learn all possible routes, and choose the best route.
2 The data link layer defines the rules that determine when a device can send data over a particular medium. Data link protocols also define the format of a header and trailer that allows devices attached to the medium to successfully send and receive data.
1 This layer typically refers to standards from other organizations. These standards deal with the physical characteristics of the transmission medium, including connectors, pins, use of pins, electrical currents, encoding, light modulation, and the rules for how to activate and deactivate the use of the physical medium.
Table 1.5 OSI Reference Model—Example Devices and Protocols
Memorize the names of the layers. You can simply memorize them, but some people like to use a mnemonic phrase to make memorization easier. In the following three phrases, the first letter of each word is the same as the first letter of an OSI layer name, in the order specified in parentheses:
- All People Seem To Need Data Processing (Layers 7 to 1)
- Please Do Not Take Sausage Pizzas Away (Layers 1 to 7)
- Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7)
Predict the data flow between two hosts across a network
Determine the path between two hosts across a network
Once you create an internetwork by connecting your WANs and LANs to a router, you’ll need to configure logical network addresses, such as IP addresses, to all hosts on the internetwork so that they can communicate across that internetwork.
The term routing is used for taking a packet from one device and sending it through the network to another device on a different network. Routers don’t really care about hosts—they only care about networks and the best path to each network. The logical network address of the destination host is used to get packets to a network through a routed network, and then the hardware address of the host is used to deliver the packet from a router to the correct destination host.
If your network has no routers, then it should be apparent that you are not routing. Routers route traffic to all the networks in your internetwork. To be able to route packets, a router must know, at a minimum, the following:
- Destination address
- Neighbor routers from which it can learn about remote networks
- Possible routes to all remote networks
- The best route to each remote network
- How to maintain and verify routing information
The router learns about remote networks from neighbor routers or from an administrator. The router then builds a routing table (a map of the internetwork) that describes how to find the remote networks. If a network is directly connected, then the router already knows how to get to it.
If a network isn’t directly connected to the router, the router must use one of two ways to learn how to get to the remote network: static routing, meaning that someone must hand-type all network locations into the routing table, or something called dynamic routing. In dynamic routing, a protocol on one router communicates with the same protocol running on neighbor routers. The routers then update each other about all the networks they know about and place this information into the routing table. If a change occurs in the network, the dynamic routing protocols automatically inform all routers about the event. If static routing is used, the administrator is responsible for updating all changes by hand into all routers. Typically, in a large network, a combination of both dynamic and static routing is used.
Figure 1.10 shows a simple two-router network. Lab_A has one serial interface and three LAN interfaces.
Looking at Figure 1.10, can you see which interface Lab_A will use to forward an IP datagram to a host with an IP address of 10.10.10.10?
FIGURE 1.10 A simple routing example
By using the command show ip route, we can see the routing table (map of the internetwork) that Lab_A uses to make forwarding decisions:
The C in the routing table output means that the networks listed are “directly connected,” and until we add a routing protocol—something like RIP, EIGRP, or the like—to the routers in our internetwork (or use static routes), we’ll have only directly connected networks in our routing table.
So let’s get back to the original question: By looking at the figure and the output of the routing table, can you tell what IP will do with a received packet that has a destination IP address of 10.10.10.10? The router will packet-switch the packet to interface FastEthernet 0/0, and this interface will frame the packet and then send it out on the network segment.
Because we can, let’s do another example: Based on the output of the next routing table, which interface will a packet with a destination address of 10.10.10.14 be forwarded from?
First, you can see that the network is sub-netted and each interface has a different mask. And I have to tell you—you just can’t answer this question if you can’t subnet! 10.10.10.14 would be a host in the 10.10.10.8/29 subnet connected to the FastEthernet0/1 interface.
Figure 1.11 shows a LAN connected to Router A, which is, in turn, connected via a WAN link to RouterB. RouterB has a LAN connected with an HTTP server attached.
FIGURE 1.11 IP routing example 1
The critical information you need to glean from this figure is exactly how IP routing will occur in this example. Okay—we’ll cheat a bit. I’ll give you the answer, but then you should go back over the figure and see if you can answer example 2 without looking at my answers.
1. The destination address of a frame, from HostA, will be the MAC address of the F0/0 interface of the RouterA router.
2. The destination address of a packet will be the IP address of the network interface card (NIC) of the HTTP server.
3. The destination port number in the segment header will have a value of 80.
That example was a pretty simple one, and it was also very to the point. One thing to remember is that if multiple hosts are communicating to the server using HTTP, they must all use a different source port number. That is how the server keeps the data separated at the Transport layer.
Let’s mix it up a little and add another internetworking device into the network and then see if you can find the answers. Figure 1.12 shows a network with only one router but two switches
FIGURE 1.12 IP routing example 2.
What you want to understand about the IP routing process here is what happens when HostA sends data to the HTTPS server:
1. The destination address of a frame, from HostA, will be the MAC address of the F0/0 interface of the RouterA router.
2. The destination address of a packet will be the IP address of the network interface card (NIC) of the HTTPS server.
3. The destination port number in the segment header will have a value of 443.
Notice that the switches weren’t used as either a default gateway or another destination. That’s because switches have nothing to do with routing. I wonder how many of you chose the switch as the default gateway (destination) MAC address for HostA? If you did, don’t feel bad—just take another look with that fact in mind. It’s very important to remember that the destination MAC address will always be the router’s interface—if your packets are destined for outside the LAN, as they were in these last two examples.
Before we move into some of the more advanced aspects of IP routing, let’s discuss ICMP in more detail, as well as how ICMP is used in an internetwork. Take a look at the network shown in Figure 1.13. Ask yourself what will happen if the LAN interface of Lab_C goes down.
Lab_C will use ICMP to inform Host A that Host B can’t be reached, and it will do this by sending an ICMP destination unreachable message. Lots of people think that the Lab_A router would be sending this message, but they would be wrong because the router that sends the message is the one with that interface that’s down is located.
FIGURE 1.13 ICMP error example
Let’s look at another problem: Look at the output of a corporate router’s routing table:
What do we see here? If I were to tell you that the corporate router received an IP packet with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3, what do you think the Corp router will do with this packet?
If you said, “The packet came in on the FastEthernet 0/0 interface, but since the routing table doesn’t show a route to network 192.168.22.0 (or a default route), the router will discard the packet and send an ICMP destination unreachable message back out interface FastEthernet 0/0,” you’re a genius! The reason it does this is because that’s the source LAN where the packet originated from.
Identify the appropriate media, cables, ports, and connectors to connect Cisco network devices to other network devices and hosts in a LAN
Select the appropriate media, cables, ports, and connectors to connect switches to other network devices and hosts
Ethernet cabling is an important discussion, especially if you are planning on taking the Cisco exams.
Three types of Ethernet cables are available:
- Straight-through cable
- Crossover cable
- Rolled cable
Straight-Through Cable
- The straight-through cable is used to connect
- Host to switch or hub
- Router to switch or hub
Four wires are used in straight-through cable to connect Ethernet devices. It is relatively simple to create this type; Figure 1.12 shows the four wires used in a straight-through Ethernet cable.
FIGURE 1.12 Straight-through Ethernet cable
Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6, and you’ll be up and networking in no time. However, remember that this would be an Ethernet-only cable and wouldn’t work with voice, Token Ring, ISDN, and so on.
Crossover Cable
The crossover cable can be used to connect
- Switch to switch
- Hub to hub
- Host to host
- Hub to switch
- Router direct to host
The same four wires are used in this cable as in the straight-through cable; we just connect different pins together. Figure 1.13 shows how the four wires are used in a crossover Ethernet cable.
Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3 and 2 to 6 on each side of the cable.
FIGURE 1.13 Crossover Ethernet cable
Rolled Cable
Although rolled cable isn’t used to connect any Ethernet connections, you can use a rolled Ethernet cable to connect a host to a router console serial communication (com) port. If you have a Cisco router or switch, you would use this cable to connect your PC running HyperTerminal to the Cisco hardware. Eight wires are used in this cable to connect serial devices, although not all eight are used to send information, just as in Ethernet networking. Figure 1.14 shows the eight wires used in a rolled cable.
These are probably the easiest cables to make because you just cut the end off on one side of a straight-through cable, turn it over, and put it back on (with a new connector, of course).
FIGURE 1.14 Rolled Ethernet cable
Once you have the correct cable connected from your PC to the Cisco router or switch, you can start HyperTerminal to create a console connection and configure the device. Set the configuration as follows:
1. Open HyperTerminal and enter a name for the connection. It is irrelevant what you name it, but I always just use Cisco. Then click OK.
2. Choose the communications port—either COM1 or COM2, whichever is open on your PC.
3. Now set the port settings. The default values (2400bps and no flow control hardware) will not work; you must set the port settings as shown in Figure 1.14.
FIGURE 1.14 Port settings for a rolled cable connection
Notice that the bit rate is now set to 9600 and the flow control is set to None. At this point, you can click OK and press the Enter key and you should be connected to your Cisco device console port.
We’ve taken a look at the various RJ45 unshielded twisted pair (UTP) cables. Keeping this in mind, what cable is used between the switches in Figure 1.15?
FIGURE 1.15 RJ45 cable questions #1
In order for host A to ping host B, you need a crossover cable to connect the two switches. But what types of cables are used in the network shown in Figure 1.16?
FIGURE 1.16 RJ45 cable questions #2
The trouble is, we have a console connection that uses a rolled cable. Plus, the connection from the router to the switch is a straight-through cable, as is true for the hosts to the switches. Keep in mind that if we had a serial connection (which we don’t); it would be a V.35 that we’d use to connect us to a WAN.