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Routing Fundamentals and Subnets

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Routing Fundamentals and Subnets

The network layer interfaces to networks and provides the best end-to-end packet delivery services to its user, the transport layer. The network layer sends packets from the source network to the destination network.



Routers are devices that implement the network service. They provide interfaces for a wide range of links and subnetworks at various speeds. Routers are active and intelligent network nodes; therefore, they can participate in managing the network. Routers manage networks by providing dynamic control over resources and supporting the tasks and goals for networks: connectivity, reliable performance, management control, and flexibility.

In addition to the basic switching and routing functions, routers have implemented a variety of value-added features that help to improve the cost-effectiveness of the network. These features include sequencing traffic based on priority and traffic filtering.

Typically, routers are required to support multiprotocol stacks, each with its own routing protocols, and to allow these different environments to operate in parallel. In practice, routers also incorporate bridging functions and can serve as a limited form of hub.

IP addressing makes it possible for data that is passing over the network media of the Internet to find its destination. Because each IP address is a 32-bit value, there are four billion different IP address possibilities. IP addresses are hierarchical addresses, like phone numbers and zip codes. They provide a better way to organize computer addresses than MAC addresses, which are 'one-dimensional' addresses (like social security numbers). IP addresses can be set in software, so they are flexible. MAC addresses are burned into hardware. Both addressing schemes are important for efficient communications among computers.

Concept Questions

Demonstrate your knowledge of these concepts by answering the following questions in the space provided.

Path determination occurs at the network layer. Routers are another type of internetworking device. These devices pass data packets among networks based on network protocol or Layer 3 information. Explain how this process works.

Routers use logical addressing to make routing decisions. When a packet is received by a router, it pulls off Layer 2 frame encapsulation and analyzes the Layer 3 header. It uses this destination address along with its routing table to determine the best path and ultimately the next hop.

Routers have the capability to make intelligent decisions as to the best path for delivery of data on the network. What criteria do routers use to make these decisions?

Routers share information with other routers, educating one another about known networks within the internetwork. The router uses this table, almost like a network roadmap. When requested to forward a packet, the router refers to this map/table and forwards the packet to the next appropriate interface and neighbor.

IP addresses are 32-bit values that are written as four octets separated with periods. To make them easier to remember, IP addresses are usually written in dotted notation with decimal numbers. IP addresses are used to identify a machine on a network and the network to which it is attached. What do dotted-decimal and hexadecimal mean?

Dotted-decimal notation allows for easier identification of the IP address. Each decimal number (octet) represents 8 individual bits. Those bits are used to represent either the host or network portion of this unique address. The subnet mask actually indicates whether the bit is representative of the host or network

Convert the following decimal numbers to hexadecimal numbers.

A. 32,014

B. 56,432

C. 57,845

D. 98,764

E. 54,462

Hex number = 7D0E

Hex number = DC70

Hex number = E1F5

Hex number = 181CC

Convert the following hexadecimal numbers to decimal numbers.

A. 23F6

B. 6AB7

C. 5FE3

D. 87CE

E. 59AC

23F6  Decimal = 9206

6AB7  Decimal = 27,319

5FE3  Decimal = 24,547

87CE  Decimal = 34,766

59AC  Decimal = 22,956

Vocabulary Exercise

Define the following terms as completely as you can. Use the online curriculum or Chapter 8 from the Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Third Edition, for help.

Address Data structure or logical convention used to identify a unique entity, such as a particular process or network device.

Broadcast address Special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all 1s.

Internet service providers (ISPs) Companies that offer services that tie together multiple network segments.

IP (Internet Protocol) Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Documented in RFC 791.

Router Network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. Occasionally called a gateway (although this definition of gateway is becoming increasingly outdated).

Segment ) Section of a network that is bounded by bridges, routers, or switches. 2) In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters. 3) Term used in the TCP specification to describe a single transport layer unit of information.

Subnet mask A 32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. Sometimes referred to simply as mask.

Subnetwork In IP networks, a network sharing a particular subnet address.

Subnetworks—Networks arbitrarily segmented by a network administrator to provide a multilevel, hierarchical routing structure while shielding the subnetwork from the addressing complexity of attached networks. Sometimes called a subnet.

Focus Questions

What type of addressing scheme does the network layer address use?

Logical addressing occurs at the network layer. The network layer address has both network and host representation; because of this, Layer 3 addresses are said to be hierarchical.

The Internet is a collection of network segments that are tied together to facilitate the sharing of information. What specific internetworking device that operates at Layer 3 interconnects networks?

Routers allow for the interconnection of several unique segments.

3. What type of determination is the process the router uses to choose a course for the packet to travel to its destination?

Path determination is the process that a router uses to choose the next hop in a path toward a packet’s ultimate destination.

At what layer does logical addressing occur?

A network address helps the router identify a path within the network cloud. The router uses the network address to identify the destination network of a packet within an internetwork. In addition to the network address, network protocols use some form of host, or node, address. For some network layer protocols, a network administrator assigns host addresses according to some predetermined network addressing plan.

5. When a computer is moved to a different network, what type of address remains the same and what type of address must be reassigned?

When you physically move a computer to a different network, the computer maintains the same MAC address, but you must assign it a new network layer address.

What is the difference between a flat addressing scheme and a hierarchical addressing scheme?

Protocols that support the network layer use a hierarchical addressing scheme that allows for unique addresses across network boundaries, along with a method for finding a path for data to travel between networks. MAC addresses use a flat addressing scheme that makes it difficult to locate devices on other networks. Hierarchical addressing, on the other hand, not only enables information flow through an internetwork, but it also provides an efficient means of doing so.




Flat addressing does not allow for the grouping of multiple devices based upon some common characteristic, such as department, floor, company, city, and so on.

In the IP header, what information does the “total length” contain?

Specifies the length of the entire IP packet, including data and header, in two bytes (16 bits).

What three pieces of information does the subnet mask give to network devices?

The subnet mask allows the host to identify the size of the network on which it is participating. The subnet mask also allows for the identification of a network address (all 0s in the Host field) and the identification of the broadcast address (all 1s in the Host field).

How many hosts can be assigned to a Class C network?

A Class C network contains 256 host addresses. The first (.0) and the last (.255) are reserved, leaving 254 host IP addresses available for assignment.
CCNA Exam Review Questions

The following questions help you prepare for the CCNA Exam. Answers appear in Appendix B, 'CCNA Exam Review Questions Answer Key.'

Which layer of the OSI model uses the Internet Protocol addressing scheme to determine the best way to move data from one place to another?

A. Physical layer

B. Data link layer

C. Network layer

D. Transport layer

Which of the following functions allows routers to evaluate available routes to a destination and to establish the preferred handling of a packet?

A. Data linkage

B. Path determination

C. SDLC interface protocol

D. Frame Relay

IP addresses are necessary for which of the following reasons?

A. To identify a machine on a network and the network to which it is attached

B. To identify a machine on a network

C. To identify the network

D. To keep track of who is on a network

Which of the following best describes a network address on the Internet?

A. All four octets in the address are different.

B. Each address is unique.

C. The first three octets can be the same, but the last one must be different.

D. Two of the four octets can be the same, but the other two must be different.

Who assigns the network portion of every IP address?

A. The local network administrator

B. The person who owns the computer

C. The Network Information Center

D. The host network administrator

The network number plays what part in an IP address?

A. It specifies the network to which the host belongs.

B. It specifies the identity of the computer on the network.

C. It specifies which node on the subnetwork is being addressed.

D. It specifies which networks the device can communicate with.

The host number plays what part in an IP address?

A. It designates the identity of the computer on the network.

B. It designates which node on the subnetwork is being addressed.

C. It designates the network to which the host belongs.

D. It designates which hosts the device can communicate with.

A Class A address is given to what sort of organization?

A. An individual

B. A medium-size company

C. A large corporation

D. A government

In a Class A address, which octets does InterNIC assign?

A. The first octet

B. The first and second octet

C. The first, second, and third octets

D. All the octets

In a Class A address, the value of the first octet can equal which of the following?

A. 0 through 127

B. 128 through 191

C. 192 through 223

D. 192 through 255

A Class B address is given to what sort of organization?

A. An individual

B. A medium-size company

C. A large corporation

D. A government

In a Class B address, which octets are assigned locally?

A. The first octet

B. The second octet

C. The second and third octets

D. The third and fourth octets

The address 129.21.89.76 is of which class?

A. Class A

B. Class B

C. Class C

D. Address not valid

A Class C address is given to what sort of organization?

A. An individual

B. A medium-size company

C. A huge corporation

D. A government

Which of the following addresses is a Class C address? (Choose all that apply.)

A. 129.219.95.193

B. 209.101.218.30

C. 151.13.27.38

D. 192.119.15.17



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