Browser Cache Settings

Most Web browsers have adjustable settings that specify how Web pages should be updated. (At any time, however, a user can manually update a Web page, typically by clicking on a button named Refresh or Reload.) For example, Internet Explorer provides several options for specifying when the browser will go back to the originating page to check for a newer version than the one stored in the browser cache. These options, which are shown in Figure 6.4, include:
• Every visit to the page—Every time you visit a page, the browser checks to see if the page has changed since the last visit. If the page has changed, the browser displays the new page and stores a copy in the cache. This option increases the time it takes for Web pages to appear between pages you have already viewed.
• Every time you start Internet Explorer—The browser checks to see if the page has changed only when the page was viewed in an earlier session. Otherwise, all visits to the same page, in the same work session, pull data from the browser cache. This option increases performance for pages you have already viewed in a single session.
• Automatically—The browser checks to see if the page has changed only when the page was viewed in an earlier session. The browser also determines if images on the page are changing infrequently over time and check for newer images even less frequently. This option provides the best performance for pages you have already viewed.
• Never—The browser pulls all previously viewed pages from the cache and never checks to see whether pages have changed. This option increases performance for pages you have already viewed. To see the most current version of a page when using this option, you will be required to refresh the page.

Most browsers also provide an option to quickly clear the cache. Deleting the cache slows performance upon each initial visit to a Web page but provides the advantage of restoring disk space that was being used to store the cached pages. 

The Protocol

The protocol portion of the URL specifies the Internet protocol used to access network resources. Most common is the HTTP protocol, which is used to access Web pages, but other Internet protocols are also supported. Table 6.1 lists many of these protocols. This lesson focuses on the HTTP protocol because it is the most common, but you do need to know how to specify the use of other Internet services in a URL. The next two lessons offer detailed information on other Internet services. 

The protocol portion of our sample URL is shown in boldface type: 

http://www.comptia.org:80/certification/inetplus/faqs.htm

Note: When you're typing Web addresses, you can usually omit the "http://" because Web browsers accept this as the default. 

The Uniform Resource Locator (URL)

The Uniform Resource Locator (URL)
The ability to easily locate and link to resources on the Web is made possible by the Uniform Resource Locator (URL)—also called an address. The general format for a URL is:
protocol://hostname:port/path/filename.ext
The following is an example address that points to the page containing Frequently Asked Questions (FAQs) about the i-Net+ certification. This page is located on CompTIA's Web server:
http://www.comptia.org:80/certification/inetplus/faqs.htm
Next, we will use this example to examine each part of a URL.

Hypertext Transfer Protocol (HTTP)

The Hypertext Transfer Protocol (HTTP) is the protocol used over TCP/IP to carry requests and responses between a Web server and a Web client. It can be said that HTTP is to the Web what TCP/IP is to the Internet.
Note: HTTP is not a secure protocol. There are, however, a couple of variations of HTTP that provide for encrypted transmission. These variations include the Secure Hypertext Transfer Protocol (SHTTP) and Hypertext Transfer Protocol Secure (HTTPS).
HTTP is a stateless protocol, which means that it cannot remember what occurred previously. (Programs that are stateful can remember what happened the last time the program was run and keep track of various configuration settings.)
HTTP uses a series of requests and responses to transfer data. Basically, the client sends a request, and the server responds to the request. This transfer of data on the Web, using HTTP, usually occurs in the following four stages:

1. Connection—The client connects to the Web server.
2. Request—The client sends a resource request message to the server.
3. Response—The server delivers a response to the client.
4. Close—The connection is closed between the client and the server.
Note: Under the original HTTP specifications, the connection closes after the server response. However, a newer version of HTTP introduces persistent connections; this version maintains the connection and continues to exchange multiple requests until the connection is explicitly closed.
In addition to HTTP, another Web standard called Hypertext Markup Language (HTML) plays a crucial role in the Web's functionality. After the client makes its request and the server responds, the client uses a Web browser to interpret and display the information

The Client/Server Model

Web services, as well as most other Internet services, use a client/server model. The server provides the services, which are usually requested by the client. A Web client typically uses a Web browser to make requests, and the server responds to those requests by delivering Web pages, for example. While end users (the clients) are generally concerned only with what's going on at their end, you, as an Internet professional, may need to be knowledgeable about both ends. Figure 6.1 shows an example of a Web server responding to client requests.

ftp

FTP (File Transfer Protocol) is the most widely used file-transfer mechanism. First defined in RFC 959, FTP runs on port 21. To use FTP to send or receive files, your systems must meet the following requirements:
• The client computer must have FTP client software.
• The user must have a user name and password on the remote system. In many cases, a user name of anonymous with no password is used.
• The remote system must be running an FTP daemon (Unix, VMS, etc.) or service (Windows NT/2000, etc.).
• Both systems must be running the TCP/IP protocol.
Figure 8.1 shows an FTP utility (WS_FTP Pro) that can transfer files between remote and local locations.
You can use FTP in either a command-line mode or a command-interpreter mode. The following options are available from the command line:
• -a—Use any local interface when binding data connections.
• -d—Enables debugging.
• -g—Disables file-name globbing (globbing permits the use of wildcard characters such as the asterisk and question mark in local file or path names).
• -i—Turns off interactive prompting during multiple file transfers.
• -n—Suppresses auto-login upon initial connection.
• -s:filename—Specifies a text file containing FTP commands; the commands will automatically run after FTP starts.
• -v—Suppresses the display of remote server responses.
• -w:buffersize—Overrides the default transfer buffer size of 4096K.

server i locate in nepal

Swap files

A swap file (or swap space or, in Windows NT, a pagefile) is a space on a hard disk used as the virtual memory extension of a computer's real memory (RAM). Having a swap file allows your computer's operating system to pretend that you have more RAM than you actually do. The least recently used files in RAM can be "swapped out" to your hard disk until they are needed later so that new files can be "swapped in" to RAM. In larger operating systems (such as IBM's OS/390), the units that are moved are called pages and the swapping is called paging.
One advantage of a swap file is that it can be organized as a single contiguous space so that fewer I/O operations are required to read or write a complete file.

In general, Windows and Unix-based operating systems provide a default swap file of a certain size that the user or a system administrator can usually change.

Subnet Design Considerations

Subnet Design Considerations


The deployment of an addressing plan requires careful thought on the part of the

network administrator. There are four key questions that must be answered before any

design should be undertaken:

1) How many total subnets does the organization need today?

2) How many total subnets will the organization need in the future?

3) How many hosts are there on the organization's largest subnet today?

4) How many hosts will there be on the organization's largest subnet in the future?

The first step in the planning process is to take the maximum number of subnets

required and round up to the nearest power of two. For example, if a organization needs

9 subnets, 23 (or 8) will not provide enough subnet addressing space, so the network

administrator will need to round up to 24 (or 16). When performing this assessment, it

is critical that the network administrator always allow adequate room for future growth.

For example, if 14 subnets are required today, then 16 subnets might not be enough in

two years when the 17th subnet needs to be deployed. In this case, it might be wise to

allow for more growth and select 25 (or 32) as the maximum number of subnets.

The second step is to make sure that there are enough host addresses for the

organization's largest subnet. If the largest subnet needs to support 50 host addresses

today, 25 (or 32) will not provide enough host address space so the network

administrator will need to round up to 26 (or 64).

The final step is to make sure that the organization's address allocation provides enough

bits to deploy the required subnet addressing plan. For example, if the organization has

a single /16, it could easily deploy 4-bits for the subnet-number and 6-bits for the host

number. However, if the organization has several /24s and it needs to deploy 9 subnets,

it may be required to subnet each of its /24s into four subnets (using 2 bits) and then

build the internet by combining the subnets of 3 different /24 network numbers. An

alternative solution, would be to deploy network numbers from the private address

space (RFC 1918) for internal connectivity and use a Network Address Translator

(NAT) to provide external Internet access.

Subnet Example #1

Given

An organization has been assigned the network number 193.1.1.0/24 and it needs to

define six subnets. The largest subnet is required to support 25 hosts.

Defining the Subnet Mask / Extended-Prefix Length

The first step is to determine the number of bits required to define the six subnets. Since

a network address can only be subnetted along binary boundaries, subnets must be

created in blocks of powers of two [ 2 (21), 4 (22), 8 (23), 16 (24), etc. ]. Thus, it is

impossible to define an IP address block such that it contains exactly six subnets. For

this example, the network administrator must define a block of 8 (23) and have two

unused subnets that can be reserved for future growth.

Since 8 = 23, three bits are required to enumerate the eight subnets in the block. In this

example, the organization is subnetting a /24 so it will need three more bits, or a /27, as

the extended-network-prefix. A 27-bit extended-network-prefix can be expressed in

dotted-decimal notation as 255.255.255.224. This is illustrated in Figure 11.
 

Prefix Length


A 27-bit extended-network-prefix leaves 5 bits to define host addresses on each subnet.

This means that each subnetwork with a 27-bit prefix represents a contiguous block of

25 (32) individual IP addresses. However, since the all-0s and all-1s host addresses

cannot be allocated, there are 30 (25 -2) assignable host addresses on each subnet.

Extended-Network-Prefix

Extended-Network-Prefix


Internet routers use only the network-prefix of the destination address to route traffic to a

subnetted environment. Routers within the subnetted environment use the extendednetwork-

prefix to route traffic between the individual subnets. The extended-networkprefix

is composed of the classful network-prefix and the subnet-number.

The extended-network-prefix has traditionally been identified by the subnet mask. For

example, if you have the /16 address of 130.5.0.0 and you want to use the entire third

octet to represent the subnet-number, you need to specify a subnet mask of

255.255.255.0. The bits in the subnet mask and the Internet address have a one-to-one

correspondence. The bits of the subnet mask are set to 1 if the system examining the

address should treat the corresponding bit in the IP address as part of the extendednetwork-

prefix. The bits in the mask are set to 0 if the system should treat the bit as part

of the host-number. This is illustrated if Figure 9.

The standards describing modern routing protocols often refer to the extended-networkprefix-

length rather than the subnet mask. The prefix length is equal to the number of

contiguous one-bits in the traditional subnet mask. This means that specifying the

network address 130.5.5.25 with a subnet mask of 255.255.255.0 can also be expressed

as 130.5.5.25/24. The / notation is more compact and easier to

understand than writing out the mask in its traditional dotted-decimal format. This is

illustrated in Figure 10.

However, it is important to note that modern routing protocols still carry the subnet

mask. There are no Internet standard routing protocols that have a one-byte field in their

header that contains the number of bits in the extended-network prefix. Rather, each

routing protocol is still required to carry the complete four-octet subnet mask.

subbnetting

In 1985, RFC 950 defined a standard procedure to support the subnetting, or division, of


a single Class A, B, or C network number into smaller pieces. Subnetting was

introduced to overcome some of the problems that parts of the Internet were beginning

to experience with the classful two-level addressing hierarchy:

- Internet routing tables were beginning to grow.

- Local administrators had to request another network number from the Internet

before a new network could be installed at their site.

Both of these problems were attacked by adding another level of hierarchy to the IP

addressing structure. Instead of the classful two-level hierarchy, subnetting supports a

three-level hierarchy. Figure 6 illustrates the basic idea of subnetting which is to divide

the standard classful host-number field into two parts - the subnet-number and the hostnumber

on that subnet.

Subnetting attacked the expanding routing table problem by ensuring that the subnet

structure of a network is never visible outside of the organization's private network. The

route from the Internet to any subnet of a given IP address is the same, no matter which

subnet the destination host is on. This is because all subnets of a given network number

use the same network-prefix but different subnet numbers. The routers within the

private organization need to differentiate between the individual subnets, but as far as the

Internet routers are concerned, all of the subnets in the organization are collected into a

single routing table entry. This allows the local administrator to introduce arbitrary

complexity into the private network without affecting the size of the Internet's routing

tables.

Subnetting overcame the registered number issue by assigning each organization one (or

at most a few) network number(s) from the IPv4 address space. The organization was

then free to assign a distinct subnetwork number for each of its internal networks. This

allows the organization to deploy additional subnets without needing to obtain a new

network number from the Internet.

In Figure 7, a site with several logical networks uses subnet addressing to cover them

with a single /16 (Class B) network address. The router accepts all traffic from the

Internet addressed to network 130.5.0.0, and forwards traffic to the interior subnetworks

based on the third octet of the classful address. The deployment of subnetting within the

private network provides several benefits:

- The size of the global Internet routing table does not grow because the site

administrator does not need to obtain additional address space and the routing

advertisements for all of the subnets are combined into a single routing table entry.

- The local administrator has the flexibility to deploy additional subnets without

obtaining a new network number from the Internet.

- Route flapping (i.e., the rapid changing of routes) within the private network does

not affect the Internet routing table since Internet routers do not know about the

reachability of the individual subnets - they just know about the reachability of the

parent network number.

Unforeseen Limitations to Classful Addressing

Unforeseen Limitations to Classful Addressing


The original designers never envisioned that the Internet would grow into what it has

become today. Many of the problems that the Internet is facing today can be traced back

to the early decisions that were made during its formative years.

- During the early days of the Internet, the seemingly unlimited address space allowed

IP addresses to be allocated to an organization based on its request rather than its

actual need. As a result, addresses were freely assigned to those who asked for

them without concerns about the eventual depletion of the IP address space.

- The decision to standardize on a 32-bit address space meant that there were only 232

(4,294,967,296) IPv4 addresses available. A decision to support a slightly larger

address space would have exponentially increased the number of addresses thus

eliminating the current address shortage problem.

- The classful A, B, and C octet boundaries were easy to understand and implement,

but they did not foster the efficient allocation of a finite address space. Problems

resulted from the lack of a network class that was designed to support mediumsized

organizations. A /24, which supports 254 hosts, is too small while a /16,

which supports 65,534 hosts, is too large. In the past, the Internet has assigned sites

with several hundred hosts a single /16 address instead of a couple of /24s

addresses. Unfortunately, this has resulted in a premature depletion of the /16

network address space. The only readily available addresses for medium-size

organizations are /24s which have the potentially negative impact of increasing the

size of the global Internet's routing table.

The subsequent history of Internet addressing is focused on a series of steps that

overcome these addressing issues and have supported the growth of the global Internet.

Unforeseen Limitations to Classful Addressing

Unforeseen Limitations to Classful Addressing


The original designers never envisioned that the Internet would grow into what it has

become today. Many of the problems that the Internet is facing today can be traced back

to the early decisions that were made during its formative years.

- During the early days of the Internet, the seemingly unlimited address space allowed

IP addresses to be allocated to an organization based on its request rather than its

actual need. As a result, addresses were freely assigned to those who asked for

them without concerns about the eventual depletion of the IP address space.

- The decision to standardize on a 32-bit address space meant that there were only 232

(4,294,967,296) IPv4 addresses available. A decision to support a slightly larger

address space would have exponentially increased the number of addresses thus

eliminating the current address shortage problem.

- The classful A, B, and C octet boundaries were easy to understand and implement,

but they did not foster the efficient allocation of a finite address space. Problems

resulted from the lack of a network class that was designed to support mediumsized

organizations. A /24, which supports 254 hosts, is too small while a /16,

which supports 65,534 hosts, is too large. In the past, the Internet has assigned sites

with several hundred hosts a single /16 address instead of a couple of /24s

addresses. Unfortunately, this has resulted in a premature depletion of the /16

network address space. The only readily available addresses for medium-size

organizations are /24s which have the potentially negative impact of increasing the

size of the global Internet's routing table.

The subsequent history of Internet addressing is focused on a series of steps that

overcome these addressing issues and have supported the growth of the global Internet.

Additional Practice with Classful Addressing

Please turn to Appendix B for practical exercises to further your understanding of

Classful IP Addressing.

Dotted-Decimal Notation

Dotted-Decimal Notation


To make Internet addresses easier for human users to read and write, IP addresses are

often expressed as four decimal numbers, each separated by a dot. This format is called

"dotted-decimal notation."

Dotted-decimal notation divides the 32-bit Internet address into four 8-bit (byte) fields

and specifies the value of each field independently as a decimal number with the fields

separated by dots. Figure 5 shows how a typical /16 (Class B) Internet address can be

expressed in dotted decimal notation.

Figure 5: Dotted-Decimal Notation

Table 1 displays the range of dotted-decimal values that can be assigned to each of the

three principle address classes. The "xxx" represents the host-number field of the

address which is assigned by the local network administrator.

Table 1: Dotted-Decimal Ranges for Each Address Class

CLASSFUL OF IP ADRRESSING

When IP was first standardized in September 1981, the specification required that each


system attached to an IP-based internet be assigned a unique, 32-bit Internet address

value. Some systems, such as routers which have interfaces to more than one network,

must be assigned a unique IP address for each network interface.

The first part of an Internet address identifies the network on which the host resides,

while the second part identifies the particular host on the given network. This created the

two-level addressing hierarchy which is illustrated in Figure 3.

Figure 3: Two-Level Internet Address Structure

In recent years, the network-number field has been referred to as the "network-prefix"

because the leading portion of each IP address identifies the network number. All hosts

on a given network share the same network-prefix but must have a unique host-number.

Similarly, any two hosts on different networks must have different network-prefixes but

may have the same host-number.

Primary Address Classes

In order to provide the flexibility required to support different size networks, the

designers decided that the IP address space should be divided into three different address

classes - Class A, Class B, and Class C. This is often referred to as "classful"

addressing because the address space is split into three predefined classes, groupings, or

categories. Each class fixes the boundary between the network-prefix and the hostnumber

at a different point within the 32-bit address. The formats of the fundamental

address classes are illustrated in Figure 4.

One of the fundamental features of classful IP addressing is that each address contains a

self-encoding key that identifies the dividing point between the network-prefix and the

host-number. For example, if the first two bits of an IP address are 1-0, the dividing

point falls between the 15th and 16th bits. This simplified the routing system during the

early years of the Internet because the original routing protocols did not supply a

"deciphering key" or "mask" with each route to identify the length of the network-prefix.

Class A Networks (/8 Prefixes)

Each Class A network address has an 8-bit network-prefix with the highest order bit set

to 0 and a seven-bit network number, followed by a 24-bit host-number. Today, it is no

longer considered 'modern' to refer to a Class A network. Class A networks are now

referred to as "/8s" (pronounced "slash eight" or just "eights") since they have an 8-bit

network-prefix.

A maximum of 126 (27-2) /8 networks can be defined. The calculation requires that the

2 is subtracted because the /8 network 0.0.0.0 is reserved for use as the default route and

the /8 network 127.0.0.0 (also written 127/8 or 127.0.0.0/8) has been reserved for the

"loopback" function. Each /8 supports a maximum of 16,777,214 (224-2) hosts per

network. The host calculation requires that 2 is subtracted because the all-0s ("this

network") and all-1s ("broadcast") host-numbers may not be assigned to individual

hosts.

Since the /8 address block contains 231 (2,147,483,648 ) individual addresses and the

IPv4 address space contains a maximum of 232 (4,294,967,296) addresses, the /8

address space is 50% of the total IPv4 unicast address space.

Class B Networks (/16 Prefixes)

Each Class B network address has a 16-bit network-prefix with the two highest order

bits set to 1-0 and a 14-bit network number, followed by a 16-bit host-number. Class B

networks are now referred to as"/16s" since they have a 16-bit network-prefix.

A maximum of 16,384 (214) /16 networks can be defined with up to 65,534 (216-2)

hosts per network. Since the entire /16 address block contains 230 (1,073,741,824)

addresses, it represents 25% of the total IPv4 unicast address space.

Class C Networks (/24 Prefixes)

Each Class C network address has a 24-bit network-prefix with the three highest order

bits set to 1-1-0 and a 21-bit network number, followed by an 8-bit host-number. Class

C networks are now referred to as "/24s" since they have a 24-bit network-prefix.

A maximum of 2,097,152 (221) /24 networks can be defined with up to 254 (28-2)

hosts per network. Since the entire /24 address block contains 229 (536,870,912)

addresses, it represents 12.5% (or 1/8th) of the total IPv4 unicast address space.

Other Classes

In addition to the three most popular classes, there are two additional classes. Class D

addresses have their leading four-bits set to 1-1-1-0 and are used to support IP

Multicasting. Class E addresses have their leading four-bits set to 1-1-1-1 and are

reserved for experimental use.

2.3.7 DSL

2.3.7 DSL
DSL, Digital Subscriber Line, uses existing copper phone lines. DSL is available only in certain areas, and you must be within a short distance of a switching station. Speeds can vary based upon type of DSL but are typically around 9Mbps (the theoretical maximum is 52Mbps). DSL is typically less expensive than even ISDN in terms of hardware, setup, and service costs, yet the need to be within a few miles of a switching station is a big deterrent.
There are several types of DSL to choose from, and not all types are available in all markets. The types available include:
• Asymmetric DSL (ADSL)—uses existing copper phone lines
• High-bit-rate DSL (HDSL)—requires two wire pairs
• ISDN DSL (IDSL)—uses existing ISDN facilities
• Rate Adaptive DSL (RADSL)—adjusts the speed based on signal quality
• Symmetric DSL (SDSL)—a version of HDSL using a single pair of wires (and providing slower rates)
• Very-high-bit-rate DSL (VDSL)—transmits over short distances; the connection rate increases as the distance decreases
To illustrate the performance possible with the different types and the way it varies, Table 2.2 shows the transmission rates and distances for various DSL implementations.

2.3.6 ATM

2.3.6 ATM
ATM, Asynchronous Transfer Mode, is a high-bandwidth switching technology developed by the ITU Telecommunications Standards Sector (ITU-TSS). ATM uses 53-byte cells for all transmissions. Because ATM cells are uniform in length, switching mechanisms can operate with a high level of efficiency. This high efficiency results in high data-transfer rates. Some ATM systems can operate at 622Mbps; a typical working speed for an ATM, however, is around 155Mbps.
The unit of transmission for ATM is called a cell. All cells are 53 bytes long and consist of a 5-byte header and 48 bytes of data. The "Asynchronous" aspect refers to the fact that transmission time slots don't occur periodically but are granted at irregular intervals. Traffic that is time-critical, such as voice or video, can be given priority over data traffic that can be delayed slightly with no ill effect. Devices communicate on ATM networks by establishing a virtual path, within which virtual circuits can be established.

2.3.5 Frame Relay

2.3.5 Frame Relay
Frame Relay, a packet-switching protocol supporting T1 and T3, was designed to support the Broadband Integrated Services Digital Network (B-ISDN). The specifications for Frame Relay address some of the limitations of X.25. As with X.25, Frame Relay is a packet-switching network service, but Frame Relay was designed around newer, faster fiber-optic networks. Unlike X.25, Frame Relay assumes a more reliable network. This enables Frame Relay to eliminate much of the X.25 overhead required to provide reliable service on less reliable networks. Frame Relay relies on higher-level network protocol layers to provide flow and error control. To use Frame Relay, you must have special Frame-Relay-compatible connectivity devices (such as Frame-Relay-compatible routers and bridges).
Frame Relay typically is implemented as a public data network and therefore is regarded as a WAN protocol. Frame Relay provides permanent virtual circuits, which supply permanent virtual pathways for WAN connections. Frame Relay services typically are implemented at line speeds from 56Kbps up to 1.544Mbps (T1). Customers typically purchase access to a specific amount of bandwidth on a Frame Relay service, for which the customer is guaranteed access.

2.3.4 X.25

2.3.4 X.25
X.25 is a packet-switching standard widely used in WANs. The X.25 standard was developed by the International Telegraph and Telephone Consultative Committee (CCITT), which has been renamed the International Telecommunications Union (ITU). The standard, referred to as Recommendation X.25, was first introduced in 1974, and it provides to networks the options of permanent or switched virtual circuits. X.25 is required to provide reliable service and end-to-end flow control. Because each device on a network can operate more than one virtual circuit, X.25 must provide error and flow control for each virtual circuit.
A big advantage of X.25 is that it is used internationally, while the major drawback is that error checking and flow control slow down X.25. Traditionally, networks utilizing it are implemented with line speeds of up to 64Kbps. These speeds are inadequate to provide most LAN services, which typically require speeds of 10Mbps or better. X.25 networks, therefore, are poor choices for providing LAN application services in a WAN environment.

2.3.3 Tx Connections

2.3.3 Tx Connections
A T1 line is a dedicated line that operates across 24 channels at 1.544Mbps. The European counterpart to this is E1, which uses 32 channels and can run at 2.048Mbps. A T2 connection (rarely used) adds more channels (96) and can operate at 6.312Mbps. A T3 line (E3 being the European equivalent) is a dedicated line of 672 channels able to run at speeds of 43Mbps. A T4 line jumps to 4,032 channels and speeds of over 274Mbps. Of the dedicated-phone-line options, T1 and T3 are the most commonly implemented.

Very few private networks require the capacity of a T3 line, and many do not even need the full capacity of a T1. The channels of a T1 or T3 line thus can be subdivided or combined for fractional or multiple levels of service. For instance, one channel of a T1 line's 24-channel bandwidth can transmit at 64Kbps. This single-channel service is called DS-0. DS-1 service is a full T1 line. DS-1C is two T1 lines, DS-2 is four T1 lines, and DS-3 is a full T3 line (equivalent to 28 T1 lines).

Bridges

2.2.2 Bridges
While hubs are used to build a network at a single site, bridges are used to build a network at two sites—or to join two networks . A bridge operates by looking at the header of the data that comes to it. If the data is for the network on which the bridge resides, the bridge leaves the data alone. If the data is for another network, the bridge gets rid of the data by sending it to a predefined location. An example would be two networks, one in New York and the other in Chicago, that have a bridge at each location (on each network). If a user sends a message in New York and it is not for another user in New York, it must be for a user in Chicago, so it is sent there. Likewise, if a user in Chicago sends a message and it is not for another user in Chicago, it must be for a user in New York.

A bridge can never be used with more than two sites. If San Francisco were added into the mix, the bridge at Chicago could not determine whether to send the message to San Francisco or to Chicago and could send it to only one location.
The biggest advantages to bridges are that they are reasonably cheap, and they work with all protocols by dropping down to and concentrating on the physical addresses of devices. Physical addresses give bridges the ability to work with NetBEUI (NetBIOS Extended User Interface) and non-routable protocols as easily as they work with TCP/IP. Remote bridges are nothing more than bridges that connect two LANs into a WAN and filter signals.
It is important to understand that a bridge—like a hub—receives every data packet sent on the network. The bridge then looks at the header and at an internal table (known as the forwarding database, or routing table) and determines if it should leave the packet alone or send the packet out to the address it has. In this capacity, a bridge is used to expand the geographic scope of the network to another location. The opposite could also be true in that a bridge could be used to divide one network into two segments to reduce traffic throughout the whole network.
To visualize the latter situation, suppose that a company has two large departments: Manufacturing and Sales. Every piece of data generated by Manufacturing is sent throughout the network, as is every piece of data generated by Sales. If the network could be divided into two segments with a bridge between Sales and Manufacturing, the network traffic could arguably be cut in half. All the Manufacturing traffic would stay on the Manufacturing segment, and all the Sales traffic would stay on the Sales segment. Data would cross over through the bridge only when Sales requested data from or sent data to Manufacturing, or vice versa.

hubs

2.2.1 Hubs
Hubs are devices used to build networks utilizing a star topology, as shown in Figure 2.1. Hubs make it easy to add workstations to the network and to reconfigure the network at any time by simply unplugging and plugging in patch cables.



Figure 2.1 With a star topology, all devices run to a central device—a hub.
The three hub types are passive, active, and intelligent. Passive hubs allow for connections and central wiring only. Active hubs amplify the signals coming in and filter out noise. Intelligent hubs provide either switching capabilities or management features.
Switching hubs provide quick routing of signals between hub ports in order to direct data where it needs to go and reduce the bandwidth of sending the data to all locations. Switching hubs are always intelligent hubs, but intelligent hubs are not always switching hubs.

In the absence of switching, a hub sends all traffic it receives to all ports.
Hubs are occasionally known as concentrators and range in size from 4 ports to 16 ports or more. Cascading allows numerous hubs to be connected to form larger networks. Where switching is employed, it is possible for a hub to perform some of the functions of a bridge—but this is typical only if multiple networks are within a limited geographic scope.

networking devices

2.2 Networking Devices
A network can be as small as two computers talking together in a peer-to-peer relationship, or as large as the Internet—with unlimited possibilities between the two ends of the spectrum. All networks, regardless of size, have the following items in common:
• An operating system on the client or host that allows for the use of networking redirectors
• A networking protocol—a common language—through which communication can take place. Every workstation must run its own stack or use the stack of a server (as in a proxy server) to be able to communicate.
• Applications that utilize the network—email, FTP, etc.
• Network interface cards (NICs) installed in each machine
• Cabling
The cabling can be of various types, or even wireless. Cable types are tested heavily in the Network+ exam. The i-Net+ exam picks up in content where Network+ leaves off and looks at the connectivity devices used between the hosts to build the network. In particular, you must know four connectivity devices—hubs, bridges, routers, and gateways—each of which is examined in the following sections.

intetnet+basic 2

8. TCP/IP is required for a client to access the Internet. Implementations of TCP/IP differ between operating systems. Microsoft implements TCP/IP as a Windows Socket DLL in the Windows operating system.
9. An FTP client can choose to download a file in different formats (binary/ASCII) and use the following commands for interacting with files:
o put—To copy a file to a remote site
o get—To retrieve a file from a remote site
o mput—To copy multiple files to a remote site
o mget—To retrieve multiple files from a remote site
10. Name resolution can translate names into addresses by using any of the following methods:
o HOSTS file—Resolves host names to IP addresses.
o LMHOSTS file—Resolves NetBIOS names to IP addresses on computers running Windows operating systems.
o DNS—The hierarchical system used on the Internet.
o WINS—The method associated with Windows NT. WINS uses a distributed database.
11. The configuration of TCP/IP can be automated through the use of DHCP (Dynamic Host Configuration Protocol) servers. Whereas static IP addresses must be manually assigned, DHCP servers can dynamically configure the assignment of client IP addresses.
12. MIME (Multipurpose Internet Mail Extensions) makes it possible to send non-ASCII files by email.
13. Cookies hold values about a user's preferences locally on the user's machine. Browsers can be configured to control the behavior of cookies. For example, cookies can be automatically accepted, or the user can be prompted before a cookie is sent from a server.
14. Application programming interfaces (APIs) serve as building blocks for creating software that interacts with the operating-system components.
15. CGI (Common Gateway Interface) is a common language used for creating server-based applications.
16. Java allows programs to be run in a Java Virtual Machine in almost any operating system.
17. XML (Extensible Markup Language) surpasses HTML (Hypertext Markup Language) in features and allows for multiple links from one hot spot.
18. Active Server Pages (ASP) allow processing to be done on the server or on the client. When processing is done on the server, the client sees only the result as pure HTML.
19. Open Database Connectivity (ODBC) allows Web servers to interact with SQL Server.
20. BinHex converts binary data into ASCII.
21. Use Shockwave to create interactive content. Because Flash uses vector-based imaging, it is ideal for creating smooth and spectacular effects.
22. GIF, JPEG, and PNG are image file formats used for Web graphics. JPEG is better suited for photographs, and GIF is better suited for simple graphics. PNG is a newer specification designed to replace GIF and avoid licensing problems.
23. GIF89, a newer GIF specification, adds support for transparency and animation.

24. The following are basic HTML tags and their functions
Opening
Tag Closing
Tag
Function
A tag common to all Web pages; used to enclose the Web page.
A tag common to all Web pages; used to enclose other tags, which will apply to the entire document.
A tag common to all Web pages; used to enclose the Web page's title.
A tag common to all Web pages; used to enclose the content of the Web page.
Allows comments to be inserted that will not be displayed by the browser.
Makes text bold.
Makes text italic.
Where # is a number from one to six—creates headings of various levels, with one being the highest.

. Creates a bulleted, unordered list

Creates a numbered, ordered list.

Creates a glossary-like definition list.

Creates a form to solicit user input.
Creates a table to organize and present information.
25. The following are the character entity codes for special characters.
Character Entity Name Entity Number
Less than (<) < <
Greater than (>) > >
Copyright (©) © ©
Registered
trademark (®) ® ®

27. The following are characteristics of the three primary classes of networks.

Class First
Octet Networks
Aailable Hosts
Available
A 1-26 126 116,777,214
B 128-191 16,384 65,534
C 192-223 2,097,152 254
28. Memorize the following default subnet masks:
o Class A—255.0.0.0
o Class B—255.255.0.0
o Class C—255.255.255.0
29. Subnetting divides the maximum number of hosts available in an IP address set into a number of subnetworks with a limited number of hosts for each. The following are examples for a Class C network:
o 255.255.255.192 provides 2 networks with 62 hosts each.
o 255.255.255.224 provides 6 networks with 30 hosts each.
o 255.255.255.240 provides 14 networks with 14 hosts each.
o 255.255.255.248 provides 30 networks with 6 hosts each.
o 255.255.255.252 provides 62 networks with 2 hosts each.
30. Class D networks are used for multicasting.
31. 127.0.0.1 is reserved for loopback.
32. Private IP addresses are used when you do not need your network to connect to the outside world.
33. SLIP is the oldest line protocol, offering no error correction or support for dynamic IP addressing. PPP replaces SLIP and allows for error correction, dynamic IP addressing, and the use of protocols other than TCP/IP.
34. Tunneling protocols include PPTP, L2F, L2TP, and IPsec. All are Layer 2 protocols, except IPsec, which is a Layer 3 protocol.
35. PPTP is newer than PPP and adds the ability to tunnel and create secure connections. One alternative to PPTP is L2F.
36. IP addresses on a host can be seen with the Ipconfig, Winipcfg, or Ifconfig utilities.
37. PING is the all-purpose connectivity diagnostic utility. It is surpassed in operation by TRACERT, which traces a packet over the various hops from a computer to a host.
38. Types of modems that can be used for connectivity include:
o Analog—The traditional modem used over the Public Switched Telephone Network (PSTN).
o Integrated Services Digital Network (ISDN)—Provides high-speed digital connectivity over a communications network optimized for data, voice, and video transmissions.
o Digital Subscriber Line (DSL)—Provides high-speed digital connectivity over upgraded telephone lines.
o Cable—Provides high-speed connection using the coaxial cable used for cable television.
39. ISDN is a digital system that provides for the simultaneous transfer of voice and data. Data speeds can reach up to 128Kbps.
40. The two main categories of DSL include ADSL and SDSL.
41. Networks can be connected via bridges, routers, brouters, or gateways.
42. Internet bandwidth technologies include T1 and its European equivalent, E1.
43. The top-level domains include:
o com—Commercial organizations
o edu—Educational organizations
o gov—Government institutions
o mil—Military groups
o net—Internet infrastructure organizations
o org—Non-profit organizations and those not covered above

i-NET+ BASICS

1. The Internet depends upon the TCP/IP protocol and its suite of features.
2. Every IP host must have a unique address—a 32-bit binary number. Every IP host must also have a subnet mask and a default gateway setting.
3. Internet Service Providers (ISPs) provide access to the Internet through Network Access Points (NAPs).
4. A Uniform Resource Locator (URL) is used to access resources on the Internet. The URL specifies the protocol used to access the re-source (such as http: for a Web page), the name of the server where the resource resides (such as www.domain.com), the port (such as :8080), and the path to the resource (such as /folder/file.htm).
5. Know the common TCP/IP ports:
o FTP—21
o Telnet—23
o SMTP—25
o HTTP (WWW)—80
o POP3—110
o NNTP—119
o LDAP—389
6. Caching allows RAM to be used instead of actual access to speed up operations. Caching can be done on the client or server. A client can also cache Web data, increasing the overall efficiency with which Web pages are retrieved later.
7. To require that a word appear in the results of a keyword search, use a plus sign before the keyword. To ensure that a word does not appear in the results, use a minus sign before the keyword.