How to Smash TCP/IP in your CCNA Exam

**Bahaa Hamam** · 03-02-2010, 17:36

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Welcome to Lesson 4
"How to Smash TCP/IP"

If you are clear with our previous Lessons we may start with Lesson 4
By the end of this session, you will have a solid grasp of the
theories that will help you understand the “Describe How a Network Works” section in my full CCNA course.

You will learn to…

• Identify the different protocols and services for each TCP/IP Layer

• Differentiate TCP from UDP

• Understand the concept of IP Address

What is TCP/IP?
TCP/IP is by far the most commonly used protocol in the networking world. It is considered as one of the major catalysts for the rapid growth of the Internet.

For this reason, knowing the TCP/IP concepts will be instrumental to ensure your success in getting CCNA Certified.

TCP/IP is a suite of specialised protocols that is based on the US government’s Department of Defence (DoD) Model.

It is the protocol of choice for both LANs and WANs because of its low operating cost. It is also highly flexible.

It can run on any platforms or combination of network operating systems and media. TCP and IP are two distinct network protocols.

However, TCP and IP are so commonly used together that TCP/IP has become a standard.

The TCP/IP Suite
TCP/IP is basically a condensed version of the OSI stack. There are only four layers, shown in the preceding table along with its OSI counterparts:

Figure 4.0 TCP/IP versus OSI

1.Application Layer
This layer is also known as the Process Layer. It corresponds to the three upper layers of the OSI Stack. It has a wide array of protocols that define how each computer’s applications will interact with others.

Now, be sure to pay attention to the protocols under this layer and be acquainted with their functions. You will definitely encounter them again during the CCNA exam.

a.Telnet
Telnet is short for “telephone network.” This protocol allows text-mode type terminal emulations. It permits a user to connect to a remote computer by logging in to a Telnet client.

Telnet can also be used to configure and manage routers and switches.

Figure 4.1 – This is a Telnet interface screen

b.File Transfer Protocol (FTP)
FTP allows us to transfer files by providing access to directories. It also lets us manage files and directories.

FTP requires user authentication before allowing FTP access. However, you can get around this by using the “anonymous” username. This type of access only allows limited access to the user.

FTP is not only a protocol, it is also an application program. Some well know FTP clients are CuteFTP, WS_FTP and FileZilla.

TFTP (Trivial FTP) is a simplified version of FTP. It is only capable of sending and receiving files.

It does not have the ability to authenticate users. It also does not have the capacity to browse through different directories like what FTP does.

c.Simple Mail Transfer Protocol (SMTP)
SMTP is used to send emails. Its counterpart is Post Office Protocol or POP3, which receives emails.

As part of your comprehensive study for CCNA exams, you should also be aware of other application protocols such as

Simple Network Management Protocol (SNMP), Domain Name Service (DNS), Hyper Text Transfer Protocol (HTTP), and Dynamic Host Configuration Protocol (DHCP)

2.Transport Layer
This layer is also known as Host-to-Host Layer. It is equivalent to the Transport Layer of the OSI Stack.

Now, focus on the two protocols residing in this layer. Both will surely be covered by the CCNA exam. Don’t worry, we will get into the bare essentials of each.

a.Transmission Control Protocol (TCP)
TCP is a full-duplex, connection-oriented protocol that ensures a reliable and error-free transfer of data. That being said, it is very complicated and costly in terms of network bandwidth usage.

TCP establishes a connection between the sending and receiving devices through a three-way handshake as shown in the figure below.

Figure 4.2 – The TCP three-way handshake

TCP is responsible for segmenting the data stream it receives from the Application Layer. It is also its job to bring segments back to the original data format on the destination device.

Here is the TCP segment format showing the header fields and data.

Figure 4.3 – The TCP Format for segmenting Data Streams

Did you notice that there are so many header fields in a TCP segment? TCP creates a lot of overhead and eats up a huge amount of bandwidth.

In exchange of this, TCP guarantees a highly reliable delivery of data.

b.User Datagram Protocol (UDP)
UDP is a connectionless protocol that serves as a faster, simplified alternative to TCP. UDP does not employ sequencing and error- checking measures.

It addresses the issues regarding TCP’s excessive overhead by significantly reducing the number of header fields.

Figure 4.4 – The UDP segment header

For your CCNA preparation, consider memorizing the table below. It summarizes the features of TCP and UDP.

Figure 4.41 – TCP versus UDP

TCP and UDP uses port numbers in order to communicate with the upper layers. Port numbers range from 0 to 1023. You should also know by heart the commonly used ports.

It can make or break your bid for a successful CCNA certification.

Figure 4.42: Port Table - Memorise the Port Numbers

3.Internet Layer
It is in this layer where data segments are converted into packets. IP selects a suitable packet size. Then it proceeds to fragment larger packets into datagrams.

IP’s importance lies in its ability to control the routing of data across the network. It also provides a single network interface to the upper layers.

As the equivalent of Network Layer in the OSI Stack, a number of TCP/IP protocols operate in the layer that we will discuss right now.

a.Internet Protocol (IP)
IP is essentially the Internet layer. All the other protocols in this layer merely exist to support it. It has the ability to oversee all the networks and devices by using what we call IP Address.

Depending on the destination IP of the packet, IP will use a routing table to decide the best path to route the data in the network.

Keep in mind that IP is an unreliable, connectionless protocol as opposed to TCP. It provides for logical addressing on the Network Layer.

When IP receives a segment from the upper layer, it fragments the segment into a datagram similar to the figure shown below.

IP then reassembles the datagrams back into segments on the receiving end.

Figure 4.5 – The IP header

b.Address Resolution Protocol (ARP)
ARP maps the IP addresses to physical or hardware (MAC) addresses. You can view the ARP cache of a Cisco router by using the Show arp command as follows:

Figure 4.6 – ARP Cache using Telnet on Cisco Router

c.Reverse ARP (RARP)
RARP resolves physical or hardware (MAC) addresses to IP addresses. This is very helpful for devices that are not capable of storing their IP settings.

There must be a RARP server on the network in order for this to work.

d.Internet Control Message Protocol (ICMP)
This protocol is mainly used for control and messaging services. It is also the protocol relating to TCP/IP troubleshooting tools such as ping (Packet Internet Groper) and traceroute.

We will be talking about much more TCP/IP network troubleshooting techniques in the coming lessons.

4.Network Access Layer
This is the lowest layer in the TCP/IP suite. It provides the same services as the Physical and Data Link layers of the OSI stack.

And, as the layer closest to the network media, it is in charge of encapsulating datagrams into a format that is manageable by the physical network.

Addressing in TCP/IP
The Internet Protocol (IP) uses a 32-bit identification number called IP Address. All network components use the IP address to identify themselves in the network.

To make it easier for us humans to read, an IP address is written in a format called dotted decimal notation.This format consists of four numbers called octets, each separated by a period.

Each octet can have a number rangingfrom 0 to 255 (2. 192.168.10.1 and 10.0.50.253 are just two of around 4.3 billion IP addresses that we can use!

Each IP address is paired with a 32-bit Subnet Mask. It informs the rest of the network about which segment or network the particular device belongs to.

Like the IP address, the subnet mask is also written in a dotted decimal notation. An IP address can be assigned to a device in two ways:

Static - IP address is manually set to a device.

DHCP (Dynamic Host Control Protocol) – Automatically assigns a unique IP address to a network device

The steps below will teach you how to assign a static IP address to a Windows computer.

1. Click the “Start” button, and then “Settings.”
2.Choose “Control Panel.”
3.Select the “Network Connections” icon from the list.
4.Click on the “Local Area Connection” icon.

This will open the “Local Area Connection Status” dialog box.

Figure 4.7 – Network Connections window

5.Click the “Properties” button on the lower left side.

6.Highlight the “Internet Protocol (TCP/IP)” option and click the “Properties” button.

Figure 4.8 – Local Area Connection properties dialog box

Select “Use the following IP address” option and type in the desired IP address.

Figure 4.9 – Internet Protocol (TCP/IP) properties

8.Click the “OK” button.

9.To verify your IP address, simply type ipconfig –all from the command prompt.

Figure 4.10 – Using Ipconfig to display IP address.

We shall be dealing with more topics about IP addresses more in the coming lessons.

We have reached the end of today’s lesson. We hope that you have increased your appreciation on TCP/IP Suite and all the protocols associated to each of its four layers.

As we have reached the halfway mark of our CCNA Training course, you should now be more conversant with all the CCNA exam objectives that we have discussed the past four days.

In the next lesson of CCNA training, we will discuss another key CCNA topic, Remote Network Connectivity. It will surely be another interesting learning experience.

Just Give One Hour Per Day to CCNA Training System.

Until then, Happy networking!