What is the subnet mask that allows 510 hosts given the IP addess

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1- The most important characteristics in an electronic cable are the following ones: impedance, attenuation and capacitance. Could you describe them?

Impedance: The resistance to the movement of electrons in an AC circuit. Its represented by the letter Z.Like resistance, its unit of measurement is the ohm, represented by Omega.

Attenuation: refers to the resistance to the flow of electrons, and why a signal becomes degraded(to fade away) as it travels along the conduit. Its unit of measurement is the db/m.

Capacitance: How much energy it is hold in the cable. Its represented by the letter C and its unit of measurement is the picofarads pF.

2- Give a physical description of the following cables: twisted pair, coaxial and optical fiber. Explain the role of each material.Twisted pair: It consists of four pairs of thin, copper wires covered in color-code plastic insulation that are twisted together. The wire pairs are twisted for two reasons. First, to provide protection against crosstalk, which is the noise generated by adjacent pairs of wires. Two magnetic fields occur in opposite directions and cancel each other out.Second, network data is sent using two wires in a twisted pair. One copy of the data is sent on each wire, and two copies are mirror images of each other (differentials signals). If the two wires are twisted together, noise seen on one wire is also seen on the other wire. When the data is received, one copy is inverted, and the two signals are then compared. In this manner the recover can filter out noise because the noise signals cancel each other.Coaxial cable, has:-Copper conductor.-Plastic Insulation.-Braided copper shielding-Outer jacket.At the centre of the cable is a solid copper conductor. Surrounding that conductor is a layer of flexible plastic insulation. A woven copper braid or metallic foil is wrapped around the insulation. This layer acts as the second wire in the cable. It also act as a shield for the inner conductor and helps reduce the amount of outside interference. Covering this shield is the outer cable jacket. The connector used on coaxial cable is the BNC, short for British Naval Connector or Bayonet Neill Concelman, connector. Fiber-Optic cable: Five parts typically make up each fiber-optic cable:-The core-The cladding-A buffer-A strengthening material-An outer jacketThe core is the light transmission element at the centre of the optical fiber, and all the light signals travel through the core. This core is typically glass made from a combination of silica and other elements. Surrounding the core is the cladding, also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface where the core and cladding meet, which keeps light in the core as it travels down the fiber. Surrounding the cladding is a buffer material, usually plastic, that helps shield the core and cladding from damage.The strengthening material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests. The final element, the outer jacket, surrounds the cable to protect the fiber against abrasions, solvents, and other contaminants. This outer jacket composition can vary depending on the cable usage.

3- What is the difference between UTP and STP? Any consequence?The difference between UnshieldTP and ShieldTP is the shielding. The shielding reduces electrical noise. This noise reduction provides a major advantage of STP over unshielded cable. However, shielded cable is more difficult to install than unshielded cable because the metallic shielding needs to be grounded. The insulation and shielding considerably increase the size, weight, and cost of the cable. Despite these disadvantages, shielded copper cable is still used.

4- When crimping RJ-45 UTP cables, how can you create straight-through and crossover cables? Describe some situations in which they are used. Crossover cables provide a network connection between two similar devices, such as computer to computer or switch to router. With crossover cables you can connect 2 computers directly. It directly connects two network devices of the same type to each other over Ethernet. Ethernet crossover cables are commonly used when temporarily networking two devices in situations where a network router, switch or hub is not present. Crossover cables have the 1st and 3rd wires crossed, and the 2nd and 6th wires crossed. Two devices in the same category use a crossover cable. As you can see as follows:

5- What is the difference between single-mode and multimode fiber-optic cable?

The part of an optical fiber through which light rays travel is called the core of the fiber. Light rays cannot enter the core of an optical fiber at all angles. The rays can enter the core only if their angle is inside the fiber's numerical aperture: likewise, one the rays have entered the fiber's core, a limited number of optical paths exist that a light ray can follow through the fiber. These optical path are called modes. If the diameter of a fiver's core is large enough so that many paths exist that light can take as it passes through the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core that allows light rays to travel along only one path (one mode) inside the fiber.

6- Describe the basic types of networks as far as their size is concerned: PAN, LAN, CAN, MAN and WAN. Give some examples.

LAN (Local Area Network): is a group of interconnected devices that is under the same administrative control. LANs allow users to have common access to data and equipment such as printers.

WAN (Wide Area Network): are networks that connect LANs in geographically separated locations. It is a network which covers a wider area. A WAN can be as small as two LANs which are connected, or as big as the Internet. The Internet is a large WAN that is composed of millions of interconnected LANs.

MAN (Metropolitan Area Network) is a computer network that usually spans a city. A MAN usually interconnects a number of local area networks (LANs) using a high-capacity backbone technology, such as fiber-optical links. A MAN typically covers an area of between 5 and 50 km diameter.

PAN: Personal Area Network (Bluetooth)

CAN: Campus Area Network (Hospitals, Universities)

7- Compare simplex, half-duplex and full-duplex transmissions.

Simplex: The capability of transmission is only one direction between a sending station and a receiving station. Broadcast television is an example of a simplex technology.

Half-duplex: A capability for data transmission in only one direction at a time between a sending station and receiving station.

Full-duplex: The capability for simultaneous data transmission between a sending station and receiving station

8- Data are delivered by means of packets in a network. Provide a technical description of "packet".

Each piece of information transmitted on an Ethernet network is sent in a packet. A packet is a chunk of data enclosed in one or more wrapped. Three parts:-HEADER: it contains instruction about the data carried in a packet.-Length of packet: is used to check if the packet arrived correctly. (The address can be IP or MAC)-Packet number: is used because you cannot receive the packets in order.-Frame: second layer (MAC)-Packet: third layer (sender’s and receivers IP address)-PAYLOAD: Contains the amount of data which is determined by the protocol. The actual data that the packet is delivering to the destination.-CRC (Cyclic Redundancy Check): is used to hash function. It’s the most common error checking used in packets.

9- Describe the OSI model.

This model consists of seven layers, and specifies particular network functions, such as addressing, flow control, error control, encapsulation, and reliable message transfer. The following sections briefly describe each layer in the OSI reference model:

Layer 7. The Application Layer. Is the layer that is closest to the user. It provides network services to the user's applications.Layer 6. The Presentation Layer. Ensures that the information that the application layer of one system sends out can be read by the application layer of another system. If necessary, the presentation layer translates among multiple data formats by using a common format. One of the more important tasks of this layer is encryption and decryption.Layer 5. The Session Layer. As its name implies, the session layer establishes, manages, and terminates sessions between two communicating hosts. The session

layer provides its services to the presentation layer. It also synchronizes dialogue between the two host's presentations layers and manages their data exchange.Layer 4: The Transport Layer: This layer is responsible for reliable network communication between en nodes. The transport layer provides mechanisms to establish, maintain, and terminate virtual circuits, transport fault detection and recovery, and information flow control.Layer 3: The Network Layer: This layer provides connectivity and path selection between two ends systems. The network layer is the layer at which routing occurs.Layer 2: The Data Link Layer: This layer provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical(as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control.Layer 1: Physical Layer: The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between the systems.

10- Describe the following application protocols in TCP/IP: SMTP, POP3, IMAP4, FTP, HTTP, HTTPS and DNS.SMTP:Simple mail transfer Protocol. The SMTP protocol transport email messages in ASCII format using TCP. When a mail server receives a message destined for a local client, it stores that message and waits for the client to collect the mail. Mail clients can collect their mail in several ways:They can use programs that access the mail server’s files directly or can use one of many network protocols. The most popular mail client protocols are Post Office Protocol Version 3(POP3) and Internet Messaging Access Protocol version 4 (IMAP4)POP3: Post Office Protocol Version 3, which uses TCP port 110, is a mail protocol that is responsible for holding e-mail until delivery. When a SMTP servers sends an e-mail message to a POP3 server, POP3 holds on to the message until a user makes a request to have the data delivered. Thus, POP3 transfers mail files from a mail server to a mail client.

IMAP4: Internet Message Access Protocol Version 4 allows a client to access and manipulate electronic mail messages on a server. IMAP4 permits manipulation remote message folders, called mailboxes, in a way that functionally equivalent to local mailboxes

FTP: is a fast connection-oriented, error free protocol that uses TCP ports 20 and 21. FTP allows data to be transferred between servers and clients. For FTP to connect to a remote server, IP address or host name must be provided. FTP must be capable of resolving IP addresses to host names to establish a connection.HTTP: which uses TCP port 80, allows clients to transfer documents that are written in Hypertext Markup Language (HTML) over the World Wide Web for display by a browser. It's the universal display language of the Internet.

HTTPS: Another protocol for transmitting data securely over the World Wide Web, which is designed to transmit individual messages securely. Technically, it is not a protocol in and of itself; rather, it is the result of simply layering the Hypertext Transfer Protocol (HTTP) on top of the SSL/TLS protocol, thus adding the security capabilities of SSL/TLS to standard HTTP communications.DNS: Is a name-resolution service that resolves (associates) host names to IP addresses. DNS keeps a record of IP addresses and host names in a process called a domain. DNS provides services along hierarchical chain, with a database design that is similar to a file tree structure. DNS also services requests for host names that cannot be resolved locally.

Large inter-networks have several levels of DNS servers to provide efficient name resolutions.

11- Compare the two main transport protocols (i.E. TCP and UDP).An advantage that UDP has over TCP is that, because it does not concentrate on establishing a connection, it can transmit more information in a smaller amount of time than TCP. TCP is useful for transmitting large amounts of data reliably, but with the penalty of large ACK overhead consuming bandwidth. UDP is useful for transmitting small amounts of data when reliability is less crucial, UDP lacks the overhead caused by ACKs.

12- Describe the following Internet protocols: ARP, RARP, ICMP and IGMP.

ARP: Determines de data link layer addresses for known IP addresses. ARP is used to bind(associate) the physical (MAC) addresses with a specific logical (IP) address. When the data packet is sent to a particular destination, ARP matches the addressing information against the ARP cache for the appropriate MAC address. If no matches are made, ARP sends a broadcast message on the network looking for the particular destination. A host responds with the correct address and sends a reply to ARP. RARP: (Reverse address Resolution Protocol)Determines de network addresses when data link layer addresses are known. A protocol in TCP/IP stack that provides a method for finding IP addresses based on MAC addresses. ICMP: Internet Control Message Protocol is a network layer protocol that reports error. When datagram delivery errors occur, ICMP reports these errors to the sender of the datagram. ICMP does not correct the encountered network problem. IGMP: The Internet Group Management Protocol (IGMP) is a communications protocol used by hosts and adjacent routers on IP networks to establish multicast group memberships.

13- Compare the role and format of IP addresses and MAC addresses.

Although IP works in a network layer and MAC works in a physical layer the role is the same. Identify every device that connects to a network or LAN. The IP addresses are controlled by the AIANA (for instance in Spain) and the MAC addresses are controlled by the IEEE. The first known as a software address and the last as a hardware address or physical address. The IP addresses are 4 bytes long and the MAC addresses are 6 bytes long.

14- How and why can you ping an IP address?

To ping you have to run cmd application and It send ICMP echo message and wait for its reply. Often used in IP networks to test the reachability of a network device.

15- Compare subnet mask classes A, B and C. How do they relate with IP addresses? The subnet mask indicates the network portion of an IP address. Usually all hosts within a LAN use the same subnet mask Class A use the first byte for the network.Class B use the first and the second byte for the network. Class C use the first, second and third byte for the network.

16- Describe in detail how TCP communication occurs: TCP 3-Way Handshake.

HANDSHAKE: Establishing a normal TCP connection requires three steps.

1. The first host (FH) sends the second host (SH) a "synchronize" (SYN) message with its own sequence number, which second host receives. 2. (SH) replies with a synchronize-acknowledgment (SYN-ACK) message with its own sequence number and acknowledgement number, which (FH) receives. 3. (FH) replies with an acknowledgment message with acknowledgement number, which (SH)receives, and doesn't need to reply to.

17- Describe the main physical topologies (e.G. Bus, star, ring and mesh) according to parameters such as cost, set-up, scalability, maintenance and performance.

BUS: a bus topology connects all the devices using a single cable. The main cable segment must end with a terminator that absorbs the signal when it reaches the end of the line or wire. If there is no terminator, the electrical signal representing the data bounces back at the

end of the wire, causing errors in the network.

STAR: Is made up of a central connection that is a device such as switch. Although a star topology costs more to implement that the physical bus topology, the advantages of a star topology make it worth the additional cost. Because each host is connected to the central device with its own cable, when the cable has a problem, only that host is affected but if a central device fails, the whole network becomes disconnected.

RING: A topology in which host are connected in the form of a ring or circle. Unlike the physical bus topology, the ring topology has no beginning or end that needs to be terminated.

MESH: Connects all nodes to each other for redundancy and fault tolerance. The advantage is that every node is connected physically to every other node, which creates a redundant connection. If any link fails, information can flow through other link to reach its destination. The disadvantage is that for anything more than a small number of nodes, the amount of media for the links and the number of the connections on the lines becomes overwhelming.

18- Explain how communication takes place between two computers within an Ethernet network. Remember that Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection). A media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for specific period of time, a device can transmit. If two devices transmit once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time.The retransmission delay when a collision occurs I the backoff.             19 Explain how communication takes place between two computers in a Token Ring network.CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:A device with a frame to send listens until Ethernet is not busy.When the Ethernet is not busy, the sender begins sending the frame.The sender listens to make sure that no collision occurred.Once the sender hears the collision, they each send a jamming signal, to ensure that all stations recognize the collision.After the jamming is complete, each sender randomizes a timer and waits that long.When each timer expires, the process starts over with Step 1- 20 How do switches work? Describe some core functions such as learning, flooding, filtering, forwarding and aging.Switches work the same way as hubs, but they can identify the intended destination of the information that they receive, so they send that information to only the computers that are supposed to receive it.The functions are:Learning: It reads the MAC and saves it to the lookup table. Now the switch knows where to find the node. (Source MAC, segment, Port, Timestamp)Flooding: Send packet to all of the segments except by which it has arrived. The correct Node sends an acknowledgement to source node.Filtering: When you have the lookup table complete, find the value MAC in the table.Forwarding: The source node consults the MAC of target node into the switch table and then sends the packet directly.Aging: The switch has a user-configurable timer that erases the entry after a certain length of time with no activity from that node. This frees up available memory resources 21- Describe the difference between static and dynamic routing. The process to finding a path to destination host is routing.  Static routing allows routers to properly route a packet from network to network based on manually configured information. Dynamic routing adjusts automatically to network topology or traffic changes. Also called adaptive routing.Static route knowledge is administered manually by a network administrator who enters it into a router's configuration. The administrator must update these static route entry annually whenever an internetwork topology change requires an update. Dynamic route knowledge works differently. After a network administrator enters configuration commands to start dynamic routing, the route knowledge automatically is updated by a routing process whenever new information is received from the internetwork. Changes in dynamic know ledge are exchanged between routers as part of the update process.-22- State some advantages and disadvantages between these two types of wireless data transmission media: infrared and radio-frequency.The radio spectrum is the part of the electromagnetic spectrum used to transmit voice, video, and data. It uses frequencies from 3 kilohertz to 300 gigahertz. Each type of wireless data communication has its advantages and drawbacks, as follows: Infrared(IR):Very high data rates and lower cost, but very short distance. Narrowband: Low data rates and medium cost. Requires a license and covers a limited distance.Spread Spectrum: Medium cost and high data rates. Limited to campus coverage.-23- Describe terms such as “extension point”, “dead spot” and “roaming” used in wireless networks.Extension point: An extension point also called repeater is an electronic device that receives a signal and retransmits it at a higher level or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.Dead spot:A dead zone is simply an area within your house, apartment, office, or any other area that’s supposed to be covered by Wi-Fi; but it doesn’t work there.Roaming:There are two definitions for wireless LAN roaming:Internal Roaming (1): The Mobile Station (MS) moves from one access point (AP) to another AP within a home network because the signal strength is too weak.External Roaming (2): The MS (client) moves into a WLAN of another Wireless Internet Service Provider (WISP) and takes their services (Hotspot).-24-Explain how communication takes place between two computers with the CSMA/CA standard in wireless networks. A carrier sensing is used, but nodes attempt to avoid collisions by transmitting only when the channel is sensed to be "idle".- 25-Compare 802.11 standards (a, b and g) according to their frequency, channels, data rate and other characteristics. Like 802.11a, 802.11g uses Orthogonal Frequency Division Multiplexing and supports 54 Mbps. However, 802.11g is not compatible with 802.11a. For one thing, 802.11g uses 2.4GHz, whereas 802.11a uses 5 GHz. On the other hand, 802.11g is backwards compatible with 802.11b. To support 802.11b, 802.11g also supports the complementary code keying (CCK) technique used in 802.11b.-26 Describe the step-by-step process to configure a WAP as an extension point. Connect the computer to one of the four LAN ports on your router.Open a web browser, type "" in the address bar and press Enter on your keyboard.Click Login in the upper right corner.The router does not ship with a password, so just click Submit.Click Use as Access Pointon the left side of the page.Select Enable. This will give you the options to set the IP Address and Subnet mask for the router. These settings should match your existing network settings. By default, the IP address will be set to and the Subnet mask will be Some of the older router models have a different setup procedure:Click LAN settings.When the LAN settings page opens, you need to make two changes.First, change the IP Address to something such as or you can use As long as the first three sets of numbers match your existing network, you will be fine.Secondly, you need to turn off the DHCP server.When finished, click Apply Changes. Congratulations! You've now set up your router as an access point.-

 27- The main problem with radio-frequency connections is interference. Bluetooth uses a technique called Frequency Hopping Spread-Spectrum (FHSS) to avoid this problem. Describe this technique.Transmissions hop from one frequency to another in random patterns. This technique enables the transmissions to hop around narrowband interference, resulting in a a clearer signal and higher reliability of the transmission. However this technology is slower, and the receiver mustuse the same pattern to decode.-28- ISDN uses the same UTP wiring as POTS, yet it can transmit data at much higher speeds. How? ISDN stands for Integrated Services for Digital Network, is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.

-29- What is the main problem for users of cable Internet? Cable modems can have a problem known in industry jargon as "flap" or "flapping". A modem flap is when the connection by the modem to the head-end has been dropped (gone offline) and then comes back online.

-30- Describe internal and external factors which can affect your Internet speed at home. Every component of any device can have an effect on your Internet speed. The network interface card, operating system, web browser, amount of available memory and other applications running on the device all affect the upload and download speeds. -31- Describe the “Caesar cipher” formula when encrypting the plaintext “SECRET” when the key is 10.It is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a some letter  fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on.The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B= 1,..., Z = 25. Encryption of a letter by a shift n can be described mathematically E(x)=(x+n) Decryption is performed similarly, D(x)=(x-n)-32- How do symmetric-key and public-key systems work?Two separate keys, one of which is secret and one of which is public. Although different, the two parts of the key pair are mathematically linked. One key locks or encrypts the plaintext, and the other unlocks or decrypts the cipher text. Neither key can perform both functions by itself. The public key may be published without compromising security, while the private key must not be revealed to anyone not authorized to read the messages.Public-key cryptography uses asymmetric key algorithms and can also be referred to by the more generic term "asymmetric key cryptography." The algorithms used for public key cryptography are based on mathematical relationships (the most notable ones being the integer factorization and discrete logarithm problems) that presumably have no efficient solution. Although it is computationally easy for the intended recipient to generate the public and private keys, to decrypt the message using the private key, and easy for the sender to encrypt the message using the public key, it is extremely difficult (or effectively impossible) for anyone to derive the private key, based only on their knowledge of the public key. This is why, unlike symmetric key algorithms, a public key algorithm does not require a secure initial exchange of one (or more) secret keys between the sender and receiver. The use of these algorithms also allows the authenticity of a message to be checked by creating a digital signature of the message using the private key, which can then be verified by using the public key. In practice, only a hash of the message is typically encrypted for signature verification purposes.Hash encoding, or hashing, ensures that messages are not corrupted or tampered with during transmission. Hashing uses a mathematical function to create a numeric value that is unique to the data. If even one character is changed, the function output, called the message digest, will not be the same. However, the function is one-way. Knowing the message digest does not allow an attacker to re-create the message. This makes it difficult for someone to intercept and change messages. Figure 16-4 illustrates the hash encoding process. The names of the most popular hashing algorithms are SHA and MD5.- 33-Compare advantages and disadvantages between symmetric-key and public-key encryptions (e.G. Security, speed and number of keys). Security: It is more secure the asymmetric-key because the private key is never sent across the network. It is less secure the public key because the key is sent across the network and the hacker can intercept the key and decrypt the information. Speed: Symmetric encryption it is faster because it uses simpler operations, such as XOR, on smaller numbers (64 or 128 bits). Asymmetric encryption usually uses complex mathematical operations, such as power and modulus, on very large numbers (2048 bits). These operations take time. Number of keys: In case that it is uses symmetric key, for a group of N people using a secret-key cryptosystem, it is necessary to distribute a number of keys equal to: In case that it is uses asymmetric key, for a group of N people using a secret-key cryptosystem, it is necessary to distribute a number of keys equal to: 2*N -34- Explain how SSL establishes encrypted communication between a server and a client. How SSL works SSL uses both symmetric and asymmetric encryption algorithms. SSL encapsulates a symmetric When server authentication is requested, SSL uses the following process:1.To request a secure page, the client uses HTTPS.2.The server sends the client its public key and certificate.3.The client checks that the certificate was issued by a trusted party (usually a trusted Certificate Authority) that the certificate is still valid, and that the certificate is related to the contacted site.4.The client uses the public key to encrypt a random symmetric encryption key and sends it to the server, along with the encrypted URL required and other encrypted HTTP data.5.The server decrypts the symmetric encryption key using its private key and uses the symmetric key to decrypt the URL and HTTP data.6.The server sends back the requested HTML document and HTTP data that are encrypted with the symmetric key.7.The client decrypts the HTTP data and HTML document using the symmetric key and displays the information.

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