CCNA

CISCO CCNA beginner guide

CCNA (Cisco Certified Network Associate) is an information technology (IT) certification from Cisco Systems. CISCO CCNA certification is an associate-level Cisco Career certification.Passing the ICND1 exam grants the candidate the Cisco Certified Entry Networking Technician (CCENT) certification.

Yes, You can search in online about CCNA. Its help to reach you networking career and save your time and money. CCNA is one of the most respected Associate level Certification in the technological world.

Syllabus cover 

1.Computer network explained

A computer network is a telecommunications network that enables sharing of resources and information. Nodes in a network are connected with each other using either cable or  wireless media and use a system of digital rules for data exchange. Data is transferred in the form of a packet, a formatted unit of data.

When you are browsing the Internet, your computer is a part of the biggest network in the world - the Internet. If you have access to the Internet at home, your devices  are part of a LAN (Local Area Network). Here is how a simple LAN looks like:

simple LAN network

In the picture above you can see that the computer is connected to the router which is then connencted to the Internet. The computer can send and receive data on the Internet using Internet protocols such as HTTP and DNS.

To connect two computers together, we would use a device called a switch. Switches serve as a central point to which all computers on a network connect to:

lan_network_switch

2.OSI reference model

The Open Systems Interconnection (OSI) model was created by the International Organization for Standardization (ISO), the world's largest developer of voluntary international standards. It is a layered model that was created to enable different networks to communicate reliably between disparate systems. The OSI model provides a framework for creating and implementing networking standards and devices and describes how network applications on different computers can communicate through the network media.

The OSI model has seven hierarchical layers. Each layer describes a different network function. The layers are:

Application

Presentation

Session

Transport

Network

Data Link

Physical

The layers are usually numbered from the last one, which means that the Physical layer is the first layer. A mnemonic can be used to remember the seven layers: Please Do Not Throw Sausage Pizza Away

Application - Away

Presentation - Pizza

Session - Sausage

Transport - Throw

Network - Not

Data Link - Do

Physical - Please

Here is a description of each layer in the OSI model:

1. Physical – defines how to move bits from one device to another. It deals with the physical characteristics of the transmission medium, describing connectors, pins, cables,  connectors, and network interface cards.

2. Data Link – combines packets into bytes and bytes into frames. Each frame has a header and a trailer. A header contains the source and destination MAC address. A trailer  contains the Frame Check Sequence field, used for errors detection. The Data Link layer is divided into two sublayers: 

Logical Link Control – used for flow control and error detection and 

Media Access Control – used for hardware addressing and controlling the access method.

3. Network – provides logical addressing used by routers for path determination. Logical addressing is used to identify a host on a network (for example, by its IP  address).

4. Transport – provides reliable or unreliable delivery and error recovery and flow control.

5. Session – determines how to establish, control and terminate a session between the two systems.

6. Presentation – defines data formats. Processes such as compression and encryption are handled at this layer

7. Application – provides a user interface and enables network applications to communicate with other network applications.

The following list shows which protocols reside on which layer:

Application – HTTP, Telnet, FTP

Presentation – MIME

Session – SSL, NetBIOS

Transport – TCP, UDP

Network – IP, ICMP

Data Link – PPP, HDLC, Ethernet

Physical – Ethernet

3.TCP/IP reference model

The TCP/IP model is the network model used by computer networks today. It was created in the 1970s by DARPA (Defense Advance Research Project Agency) as an open, vendor-neutral, public networking model. Just like the OSI reference model, the TCP/IP model provides general guidelines for designing and implementing network protocols.

The TCP/IP model has fewer layers than the OSI model, just four. These layers describe different network functions and have their own standards and protocols. The layers are:

Application

Transport

Internet

Link

The Link layer is sometimes referred to as the Network access layer. The Transport layer is sometimes called the Host-to-Host layer.

Here is a brief description of each layer:

Link - defines the protocols and hardware required to deliver data across a physical network.

Internet - defines the protocols for the logical transmission of packets over the network.

Transport - defines protocols for setting up the level of transmission service for applications. This layer is responsible for reliable transmission of data and the the error-free delivery of packets.

Application - defines protocols for node-to-node application communication and provide services to the application software running on a computer.

Differences between the TCP/IP model and OSI model

The TCP/IP model and the OSI model are similar in concept, but have a different number of layers, sometimes with different names:

tcp ip osi model comparison

As you can see from the picture above, the Application, Presentation, and Session layers of the OSI model are merged in only one layer - the Application layer, in the TCP/IP model. The Physical and Data Link layers of the OSI model are merged into one layer, the Link layer, in the TCP/IP model.

The following list shows which protocols reside on which layer:

Application - HTTP, POP3, SMTP

Transport - TCP, UDP

Internet - IP

Link - Ethernet, PPP

4.Data encapsulation

The term encapsulation describes a process of putting headers (and sometimes trailers) around some data. Each layer adds its own header (Data Link protocols also add a trailer) to  the data supplied by the higher layer. This process can be explained with the five-layer TCP/IP model (the Link layer of the TCP/IP model is sometimes divided into two  layers, Data Link and Physical, hence the five-layer TCP/IP model), with each step corresponding to the role of each layer.

Consider the following example. Let's say that you are browsing a web site. The web server on which the website is hosted will create and encapsulate the application data with the Application layer headers. For example, the HTTP OK message will be stored in the header, followed by the requested content of the web page.

The encapsulated data will then be  forwarded to the next layer, the Transport layer. The Transport layer will add its own header around the encapsulated data. In the case of the TCP protocol, this can be something like the source and destination ports or the sequence number.

The data will then be forwarded to the next layer - the Network layer. The Network layer will  store the source and destination IP addresses in its header and forward the data to the Data Link layer. The Data Link layer is the only layer that uses both the header and trailer.

The data is then sent through a  physical network link.

Here is a graphical representation of these five steps:

encapsulation process

Each data packet (header + encapsulated data) defined by a particular layer has a specific name:

Frame - encapsulated data defined by the Network Access layer. A frame can have both header and trailer.

Packet - encapsulated data defined by the Network layer. A header contains the source and destination IP addresses.

Segment - encapsulated data defined by the Transport layer. Information such as the source and destination ports or sequence and acknowledgment numbers are included in the header.

The term decapsulation describes a process of removing headers and trailers as data passes from a lower to the upper layer. This process transpires on a computer that is receiving data.

5.Data encapsulation in the OSI model

Data encapsulation in the OSI model

In the previous lesson we have learned that the term encapsulation describes the process of putting headers (and sometimes trailers) around some data. Like with the TCP/IP layers, each OSI layer asks for services from the next lower layer. The lower layer encapsulates the higher layer’s data between a header (Data Link protocols also add a trailer). While the TCP/IP model uses terms like segment, packet and frame to refer to a data packet defined by a particular layer, the OSI model uses a different term: protocol data unit (PDU).

A PDU represent a unit of data with headers and trailers for the particular layer, as well as the encapsulated data. Since the OSI model has 7 layers, PDUs are numbered from 1 to 7, with the Physical layer being the first one. For example, the term Layer 3 PDU refers to the data encapsulated at the Network layer of the OSI model.

Here is a graphical representation of the PDUs in the OSI model:

enapsulation pdu

6.Local Area Network (LAN)

The term local area network (LAN) is used to describe a network of devices inside a limited area (in the same room, building...). A typical SOHO (small office/home office) LAN  consist of PCs, printers, switches, routers, and cabling that connects all these devices together. The following figure shows a typical LAN:

typical lan

This type of network is usually capable of achieving a high data transfer rate of (up to 10 Gbps) at relatively low cost. The twisted-pair cabling is commonly used in LANs for connections between end user devices and switches, while fiber-optic cabling is used for links between network devices, such as switches and routers.

Some of the LAN technologies are Ethernet, Token Ring and FDDI. Ethernet is by far the most popular wired LAN technology. It defines wiring, signaling, connectors, frame formats, protocol rules, etc. Most modern LANs also support the wireless LAN (WLAN) technology, defined by the IEEE 802.11 standards. WLANs use radio waves instead of wires or cables for links between devices.

7.What is Ethernet?

The term Ethernet refers to an entire family of standards that define wiring, signaling, connectors, frame formats, protocol rules, etc. Ethernet is standardized by the Institute of Electrical and Electronics Engineers (IEEE) as the 802.3 standard. The standard defines several wiring variants, such as coaxial, twisted pair and fiber optic cabling. Coaxial cables are rarely used anymore, while twisted pair cables are usually used in SOHO environments. Optical fibers are the most expensive option, but they allow longer cabling distances and greater speeds.

Ethernet uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and supports speeds up to 100 Gbps. It is by far the most popular LAN technology today.

The term Ethernet LAN refers to a combination of computers, switches, and different kinds of cables that use the Ethernet standard to communicate over the network.

8.Ethernet frame

We have already learned that encapsulated data defined by the Network Access layer is called an Ethernet frame. An Ethernet frame starts with a header, which contains the source and destination MAC addresses, among other data. The middle part of the frame is the actual data. The frame ends with a field called Frame Check Sequence (FCS).

The Ethernet frame structure is defined in the IEEE 802.3 standard. Here is a graphical representation of an Ethernet frame and a description of each field in the frame:

ethernet frame

Preamble - informs the receiving system that a frame is starting and enables synchronisation.

SFD (Start Frame Delimiter) - signifies that the Destination MAC Address field begins with the next byte.

Destination MAC - identifies the receiving system.

Source MAC - identifies the sending system.

Type - defines the type of protocol inside the frame, for example IPv4 or IPv6.

Data and Pad - contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).

FCS (Frame Check Sequence) - contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data

9.MAC address

Ethernet uses MAC (Media Access Control) addresses to uniquely identify a host in an Ethernet environment. Every Ethernet network interface card (NIC) has a MAC address burned in its firmware, which is why MAC addresses are sometimes known as hardware addresses.

MAC addresses are 6 Bytes (48 bits) long. Every network card manufacturer gets a universally unique 3-byte code called the Organizationally Unique Identifier (OUI). Manufacturers agree to give all NICs a MAC address that begins with the assigned OUI. The manufacturer then assigns a unique value for the last 3 bytes, which ensures that every MAC address is globaly unique.

In the following picture we can see the structure of a MAC address:

mac address format

MAC addresses are usually written in the form of 12 hexadecimal digits. For example, this is a valid MAC address: D8-D3-85-EA-1B-EE. Each hexadecimal character is 4 bits long, so the first six hexadecimal characters represent the vendor (in this case, Hewlett Packard).

How to find the MAC address of your computer

You can find out the MAC address your computer is using. The process depends on your operating system:

Windows

Go to the Command Prompt (Start - Programs - Accessories - Command Prompt on Windows XP, for newer versions of Windows, just type cmd from the Start screen). Once inside the Command Prompt, type the ipconfig/all command. The MAC address is shown in the Physical Address field:

mac address windows

Linux

Go to the shell and type the ifconfig command. The MAC address should be listed as HWaddr:

mac address linux

10.Unicast, multicast, broadcast addresses

Three types of Ethernet addresses exist:

unicast addresses - represents a single LAN interface. A unicast frame will be sent to a specific device, not to a group of devices on the LAN.

multicast addresses - represents a group of devices in a LAN. A frame sent to a multicast address will be forwarded to a group of devices on the LAN.

broadcast addresses - represents all device on the LAN. Frames sent to a broadcast address will be delivered to all devices on the LAN.

The broadcast address has the value of FFFF.FFFF.FFFF (all binary ones). The switch will flood broadcast frames out all ports except the port that it was received on.

Multicast frames have a value of 1 in the least-significant bit of the first octet of the destination address. This helps a network switch to distinguish between unicast and multicast addresses. One example of an Ethernet multicast address would be 01:00:0C:CC:CC:CC, which is an address used by CDP (Cisco Discovery Protocol).

11.Half and full duplex

In telecommunication, a duplex communication system is a point-to-point system of two devices that can communicate with each other in both direction. Two types of duplex communication systems exist in Ethernet environments:

half-duplex - a port can send data only when it is not receiving data. In other words, it cannot send and receive data at the same time. Network hubs run in half-duplex mode in order to prevent collisions. Since hubs are rare in modern LANs, the half-duplex system is not widely used in Ethernet networks anymore.

full-duplex - all nodes can send and receive on their port at the same time. There are no collisions in full-duplex mode, but the host NIC and the switch port must support  the full-duplex mode. Full-duplex Ethernet uses two pairs of wires at the same time instead of a single wire pair like half-duplex.

Each NIC and switch port has a duplex setting. For all links between hosts and switches, or between switches, the full-duplex mode should be used. However, for all links connected to a LAN hub, the half-duplex mode should be used in order to prevent a duplex mismatch that could decrease network performance.

Reference 

Cisco published book and 

Cisco networking applications

Cisco 200- 301 book by Wendell odom

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