VERTULIZATION IN CLOUD COMPUTING

FOR CCNA PRACTIC SOFTWARE

WHAT IS APIPA[ Automatic Private IP Addressing]

 

     

  APIPA A feature of Microsoft Windows, APIPA is a DHCP failover mechanism for local networks. With APIPA, DHCP clients can obtain IP addresses when DHCP servers are non-functional. APIPA exists in all modern versions of Windows except Windows NT.

When a DHCP server fails, APIPA allocates IP addresses in the private range 169.254.0.1 to 169.254.255.254. Clients verify their address is unique on the network using ARP. When the DHCP server is again able to service requests, clients update their addresses automatically.
In APIPA, all devices use the default network mask 255.255.0.0 and all reside on the same subnet.

APIPA is enabled on all DHCP clients in Windows unless the computer's Registry is modified to disable it. APIPA can be enabled on individual network adapters.

WHAT IS DORA PROCESS ?

DORA STAND FOR   DISCOVER OFFER REQUEST AND ACKNOWLEDGEMENT.

     when we install a dhcp server into our network then dhcp server works on the basis of dora process.

first dhcp server sends a hello message in to the network to discover the clients pc and when any client pc found in the network then , dhcp server offers the IP to client pc. When client pc select any IP from dhcp server then client pc request for selected IP to dhcp server then dhcp server provide that IP to client pc and both send ackonledgement to each other.

This process is called DORA process on the basis of this process DHCP server works to provide IP's dynamically to client pc's in network.

Most Important Command of Router Configuration

How to configure WDS function on TP-LINK Routers

To setup WDS with TP-LINK 11N routers, the following steps are required:

Step 1

Log into 11N router’s management page.

Step 2

Go to Wireless -> Wireless Settings. Check Enable WDS (Enable WDS bridging). Then the page will show as below.

Step 3

The SSID on the top of the page is the wireless network name of this router. You can name whatever you like.

Step 4

Click Search/Survey. On the pop-up window, find the SSID of your root AP, and hit Connect.

Step 5

The root AP’s SSID and BSSID (MAC Address) will be filled in automatically. Then please input the wireless security settings tomatch the ones on the root AP. Click on Save.

Step 6

Go to Wireless Security page to secure 11N router itself. The Encryption settings here could be different with your root router.

Step 7

Click DHCP-> DHCP Settings page. Choose Disable DHCP Server, and click Save button.

Step 8

Please go to System Tools-> Reboot page to reboot the unit.

Step 9

All settings required by WDS function are completed. You can make a simple check by the Ping utility such as command prompt. For how to use Ping, please refer to how to use Ping command. If the ping proceeds successfully (which means the WDS performs properly), you will see the similar screen as below:

NETWORKING CABLES

What is Network Cabling?

Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network.

Type of cables-

·        Twisted Pair

·        Coaxial Cable

·        Fiber Optic Cable

Twisted-Pair Cable-

Twisted-pair cable is a type of cabling that is used for telephone communications and most modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields. Twisting the wires can enhance this cancellation effect. Using cancellation together with twisting the wires, cable designers can effectively provide self-shielding for wire pairs within the network media.
Two basic types of twisted-pair cable exist: unshielded twisted pair (UTP) and shielded twisted pair (STP). The following sections discuss UTP and STP cable in more detail.

 Unshielded Twisted-Pair Cable-

The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated six categories of wire (additional categories are emerging).

Shielded Twisted-Pair Cable-

Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire twisting. Each pair of wires is wrapped in a metallic foil .The four pairs of wires then are wrapped in an overall metallic braid or foil, usually 150-ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the cable (EMI and RFI). STP usually is installed with STP data connector, which is created especially for the STP cable. However, STP cabling also can use the same RJ connectors that UTP uses. Although STP prevents interference better than UTP, it is more expensive and difficult to install. In addition, the metallic shielding must be grounded at both ends. If it is improperly grounded, the shield acts like an antenna and picks up unwanted signals. Because of its cost and difficulty with termination, STP is rarely used in Ethernet networks. STP is primarily used in Europe.

Coaxial Cable-

Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.

Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.

Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being 200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable has been popular in school networks, especially linear bus networks.

Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length being 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult to install.

Fiber Optic Cable-

Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify. 10BaseF refers to the specifications for fiber optic cable carrying Ethernet signals.

The center core of fiber cables is made from glass or plastic fibers. A plastic coating then cushions the fiber center, and kevlar fibers help to strengthen the cables and prevent breakage. The outer insulating jacket made of Teflon or PVC.

(UDP) User Datagram Protocol

                           

                           If you were to compare User Datagram Protocol (UDP) with TCP, the former is basically the scaled-down economy model that’s sometimes  referred to as a thin protocol. Like a thin person on a park bench, a thin protocol doesn’t take up a lot of room—or in this case, much bandwidth on a network.

UDP doesn’t offer all the bells and whistles of TCP either, but it does do a fabulous job of transporting information that doesn’t require reliable delivery—and it does so using far fewer network resources. (UDP is covered thoroughly in Request for Comments 768.)

There are some situations in which it would definitely be wise for developers to opt for UDP rather than TCP. One circumstance is when reliability is already handled at the Process/Application layer. Network File System (NFS) handles its own reliability issues, making the use of TCP both impractical and redundant. But ultimately, it’s up to the application developer to decide whether to use UDP or TCP, not the user who wants to transfer data faster.

UDP does not sequence the segments and does not care in which order the segments arrive at the destination. Rather, UDP sends the segments off and forgets about them. It doesn’t follow through, check up on them, or even allow for an acknowledgment of safe arrival—complete abandonment. Because of this, it’s referred to as an unreliable protocol. This does not mean that UDP is ineffective, only that it doesn’t handle issues of reliability.

Further, UDP doesn’t create a virtual circuit, nor does it contact the destination before delivering information to it. Because of this, it’s also considered a connectionless protocol. Since UDP assumes that the application will use its own reliability method, it doesn’t use any. This gives an

application developer a choice when running the Internet Protocol stack: TCP for reliability or UDP for faster transfers.

So, it is important to remember how this works because if the segments arrive out of order (very common in IP networks), they’ll just be passed

up to the next OSI (DoD) layer in whatever order they’re received, possibly resulting in some seriously garbled data. On the other hand, TCP

sequences the segments so they get put back together in exactly the right order—something UDP just can’t do..

UDP Segment Format

Source port Port number of the application on the host sending the data

Destination port Port number of the application requested on the destination host

Length Length of UDP header and UDP data

Checksum Checksum of both the UDP header and UDP data fields

Data Upper-layer data

UDP, like TCP, doesn’t trust the lower layers and runs its own CRC. Remember that the Frame Check Sequence (FCS) is the field that houses

the CRC, which is why you can see the FCS information.

TCP(Transmission Control Protocol)

Transmission Control Protocol (TCP)

         Transmission Control Protocol (TCP) takes large blocks of information from an application and breaks them into segments. It numbers and sequences each segment so that the destination’s TCP stack can put the segments back into the order the application intended. After these

segments are sent, TCP (on the transmitting host) waits for an acknowledgment of the receiving end’s TCP virtual circuit session, transmitting those that aren't acknowledged.

Before a transmitting host starts to send segments down the model, the sender’s TCP stack contacts the destination’s TCP stack to establish a connection. What is created is known as a virtual circuit. This type of communication is called connection-oriented. During this initial handshake,

the two TCP layers also agree on the amount of information that’s going to be sent before the recipient’s TCP sends back an acknowledgment.

With everything agreed upon in advance, the path is paved for reliable communication to take place.

TCP is a full-duplex, connection-oriented, reliable, and accurate protocol, but establishing all these terms and conditions, in addition to error checking, is no small task. TCP is very complicated and, not surprisingly, costly in terms of network overhead. And since today’s networks are much more reliable than those of yore, this added reliability is often unnecessary. Most programmers use TCP because it removes a lot of programming work; however, real-time video and VoIP use UDP because they can’t afford the overhead.

TCP Segment Format

Since the upper layers just send a data stream to the protocols in the Transport layers, I’ll demonstrate how TCP segments a data stream and

prepares it for the Internet layer. When the Internet layer receives the data stream, it routes the segments as packets through an inter network. The

segments are handed to the receiving host’s Host-to-Host layer protocol, which rebuilds the data stream to hand to the upper-layer applications or

protocols.

Source port The port number of the application on the host sending the data. (Port numbers will be explained a little later in this section.)

Destination port The port number of the application requested on the destination host.

Sequence number A number used by TCP that puts the data back in the correct order or retransmits missing or damaged data, a process

called sequencing.

Acknowledgment number The TCP octet that is expected next.

Header length The number of 32-bit words in the TCP header. This indicates where the data begins. The TCP header (even one including

options) is an integral number of 32 bits in length.

Reserved Always set to zero.

Code bits/flags Control functions used to set up and terminate a session.

Window The window size the sender is willing to accept, in octets.

Checksum The cyclic redundancy check (CRC), because TCP doesn’t trust the lower layers and checks everything. The CRC checks the

header and data fields.

Urgent A valid field only if the Urgent pointer in the code bits is set. If so, this value indicates the offset from the current sequence number, in

octets, where the segment of non-urgent data begins.

Options May be 0 or a multiple of 32 bits, if any. What this means is that no options have to be present (option size of 0). However, if any

options are used that do not cause the option field to total a multiple of 32 bits, padding of 0s must be used to make sure the data begins on a

32-bit boundary.

Data Handed down to the TCP protocol at the Transport layer, which includes the upper-layer headers.

HOW TO SETTING UP SECURE SHELL (SSH) ON A CISCO ROUTERS

                              Instead of Telnet, you can use Secure Shell, which creates a more secure session than the Telnet application that uses an unencrypted data
stream. Secure Shell (SSH) uses encryption keys to send data so that your username and password are not sent in the clear.
Here are the steps to setting up SSH:

1. Set your hostname:
Router(config)#hostname Todd

2. Set the domain name (both the hostname and domain name are required for the encryption keys to be generated):
Todd(config)#ip domain-name Lammle.com

3. Set the username to allow SSH client access
Todd(config)#username Todd password Lammle

4. Generate the encryption keys for securing the session:
Todd(config)#crypto key generate rsa general-keys modulus ?
<360-2048> size of the key modulus [360-2048]
Todd(config)#crypto key generate rsa general-keys modulus 1024
The name for the keys will be: Todd.Lammle.com
% The key modulus size is 1024 bits
% Generating 1024 bit RSA keys, keys will be non-exportable...[OK]
*June 24 19:25:30.035: %SSH-5-ENABLED: SSH 1.99 has been enabled

5. Enable SSH version 2 on the router; although this isn’t mandatory it is highly suggested:
Todd(config)#ssh version 2

6. Connect to the VTY lines of the router:
Todd(config)#line vty 0 1180

7. Last, configure SSH and then Telnet as access protocols:
Todd(config-line)#transport input ssh telnet
                     If you do not use the keyword telnet at the end of the command string, then only SSH will work on the router. I am not suggesting you use either
way, but just understand that SSH is more secure than Telnet.