How The Internet Started?
How The Internet Started?
How The Internet Started?
REGION III
Schools Division of Cabanatuan City
Maharlika Highway, Cabanatuan City
10
Jayson M. Santos,LPT (Teacher I, Camp Tinio National High School)
To You, Learner:
Welcome to ICT 10 (Web Programming) Self-Learning Module (SLM) on How the Internet
works. This module was made to give you with fun and meaningful time for guided and inde-
pendent learning at your convenience. You will be able to process the contents of this Module
while being an active learner.
This Module was collaboratively developed and reviewed by the teachers, school heads
and supervisors of DepEd Division of Cabanatuan City to assist you in helping the learners meet
the standards set by the K to 12 Curriculum while overcoming their different constraints in
schooling. As a facilitator, you are expected to orient the learners on how to use this module.
You also need to keep track of the learners' progress while allowing them to manage their own
learning. Furthermore, you are expected to encourage and assist the learners as they do the
tasks included in the module.
This module deals with skills and knowledge required to understand the issue of child
sexual exploitation by tourists and apply simple protective measures which are applicable and
appropriate for staff working in the hotel and travel industries. This will be the source of infor-
mation for them to acquire knowledge and skills in this particular competency independently
and at their own pace, with minimum supervision or help from their Teacher. It covers the infor-
mation and different activities to develop and assess the desirable values, skills and under-
standing of the learners to acquire the competency in promoting products and services to cus-
tomers . The goal of this module is the development of practical skills. To gain these skills, you
must learn the concepts and theory. Provisions for practical applications to real life situation are
also included for lifelong learning. This self-learning module designed to accomplish for one
week.
OBJECTIVES
COMPETENCY
2
WHAT DO YOU ALREADY KNOW?
Pre-Test Direction: Choose the letter of the best answer and write your answer on separate sheet
of paper.
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WHAT DO YOU ALREADY KNOW?
10 What is a two-part collection of information that is the basis for the internet communica-
tion?
I believe a scenario or case-study approach is useful to understanding general ideas. This document in-
cludes a scenario involving the loading of web pages.
Suppose you are using a web browser and click on a link to some distant web page. To you, as a user,
the computer displays the URL for the page in the location box on the browser toolbar and sits for a mo-
ment or two perhaps showing some sign of activity, and then begins displaying the contents of the new
page. Underneath this simple change a number of computers have been performing many actions nec-
essary for accomplishing the task.
There are several major steps involved in getting and displaying that web page. First, the IP address of
the textual URL must be ascertained. Then, the ground rules for communication must be established be-
tween your computer's browser (the client) and the computer that hosts the web site (the server). Next,
the client requests the desired web page and has the request evaluated by the server. If possible, the
server fulfills the request by sending the necessary file(s) to the client. Finally, the client begins rendering
and displaying the web page. It is likely that several files (CSS, images, scripts, etc.) are required and
must be requested using the process outlined below.
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Step 1
The first step is to determine the IP address of the server hosting the web page that is being sought.
The client connects to the local DNS server (supplied by your local organization or ISP) asking for an IP ad-
dress for the remote server. If the IP address is available there, it is supplied; if not:
x your local DNS server connects to the whois database server seeking the IP address for the prima
ry nameserver of the remote server’s domain.
X your local DNS server connects to the domain's nameserver to find the IP address for the web serv
er in question and get the IP address (if it exists)
X your DNS server reports back with the IP address of the desired server
It is at this point that you might get a "server not found" error message. The message is generated by
your browser locally when either there was no entry for the requested server found in the DNS server
hierarchy or some component of the DNS server hierarchy was not available.
Step 2
Establishing the connection between the client and server involves tasks similar to the transfer of the data.
This process is described in detail below and then referred to later on. Your browser has a component that
implements TCP actions. The connection establishment begins with that component. (The process de-
scribed below is a three-way handshake.)
The TCP component built into browser (client) software creates a TCP synchronization (SYN) packet con-
sisting of a header (source port, destination port, sequence number, some other overhead information,
and a checksum) and no data (though the packet has a slot for data)
The packet is sent out on/to the LAN; the hub/switch/router notes that destination is beyond LAN and
sends the packet to the next higher level within the local area. Eventually, the router to the IP (Internet
Protocol) world is reached.
X the relevant ethernet packet info is copied into the IP packet header (e.g., source & destination
IP addresses, TCP packet info, ...)
The IP packet is sent to your ISP which will forward it (perhaps through a network of routers) to its NAP for
an NSP.
The IP packet is sent through the NSP’s router network to the NAP of the web server's ISP.
The IP packet is sent through the server’s ISP router network to the web server'sNAP.
the IP packet information is discarded and (if necessary) an ethernet (or other local network protocol)
packet is created (as above). if the server is on a LAN, the packet is sent on; a hub/switch/router notes
that destination is inside its LAN and sends packet to next lower level within the local area. Eventually, the
LAN containing the server is reached.
It creates a synchronization and acknowledgment (SYN-ACK) packet containing a header with source
port, destination port, sequence/synchronization number, acknowledgment number, some other over-
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head information and a checksum, and no data
The SYN-ACK packet is sent to client making the request—this is essentially the same process as above but
in reverse
the client acknowledges the acknowledgment—again, using essentially the same process as above. Thus,
both ends have acknowledged the way in which they will communicate. Note that it is possible for this
packet to serve double duty, i.e., to include both the acknowledgment and (as data in the packet) the
requested URL.
There are several additional points about this process that are worth knowing.
The 'local" communication could have been a single step on both ends, i.e., the client communicating
directly with its ISP and the server communicating directly with its ISP. Alternatively, the server could have
been within the local collection of LANs or even on the same LAN as the client and no IP packet would
have been needed.
The packet is received, stored temporarily, examined for transmission (checksum) correctness and to de-
termine the addressee, and then passed along at each router encountered along the path. Routers be-
yond the LAN will be communicating with their neighbors to maximize network efficiency.
Packets have a limited life span. If not delivered before its time-to-live counter is counted down, a packet
will be dropped (not sent on).
Most Internet communication uses the concept of timing-out. If an anticipated action does not occur
within the expected amount of time, the process is begun again, an error message is displayed, or noth-
ing further happens (with respect to that particular action).
Much Internet traffic uses clear-text and therefore can be read by anyone who intercepts it.
Step 3
The next major step involves your browser requesting a web page/file. This process is very similar to that
above except that the TCP packet will now have data—the URL of the requested page. A single packet
will be created and sent to the server.
the browser (application software) creates a TCP "segment"/packet for the request with a header, as
above, and data (the HTTP request for a particular web page).
The packet is sent out on the LAN; the hub/switch/router notes that the destination is beyond the LAN and
sends the packet to the next higher level within the local area. Eventually, the router to the IP world is
reached.
– the ethernet packet info is copied into IP packet header (e.g., source & destination IP addresses,
TCP packet info, ...)
The IP packet is sent to the ISP which will forward it (perhaps through a network of routers) to its NAP for a
NSP.
The IP packet continues through the NSP router network to the NAP of the web server's ISP.
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The IP packet is sent through ISP router network to web server's NAP
The IP packet information is discarded and (if necessary) an ethernet (or other local network protocol)
packet is created (as above)
if the server is on a LAN, the packet is sent on; a hub/switch/router notes that destination is inside its LAN
and sends packet to next lower level within the local area. Eventually, the LAN containing the server is
reached the web server receives the request
Step 4
The server examines its file system and if the requested file exists and is available it will be prepared for
transmission to the client. If it does not exist (or some part of the URL/path has been mistyped) or has not
been made publicly readable by the owner of the file, an error message will be sent instead of the file. To
transmit the file, the web server TCP software subdivides the file and creates TCP packets and: or each
packet:prepares a header (source address, destination address, sequence number, acknowledgment
number, some other overhead information and a checksum) and includes data (a chunk of the file be-
ing sent)
sets a time check/alarm (if the packet is not acknowledged within a specified time, it will automatically
be resent) passes it along to/through the LAN or router software for appropriate packaging and routing
to the ISP and NSP (as described above)
when/if a receipt acknowledgment is received from the client, the time check/alarm associated with a
packet is disabled/deleted
when/if a time check/alarm is recognized, the associated packet is resent [Packets will be lost when they
are sent to a router that does not have sufficient buffer space to store it. This will typically be due to the
busy-ness of the net. Packets will also be lost if it is determined that garbling has occurred during transmis-
sion which is determined by calculating a checksum on the received data and comparing it to the trans-
mitted checksum.]
Step 5
The client browser receives the packets and combines them into a file. The sequence numbers in the
headers are used to tell the order in which they should appear. The browser then begins to render the
HTML file and display it appropriately. While doing so, it is likely that additional files will be needed, i.e.,
CSS, images, scripts, etc. That will cause additional requests to the server. These additional requests would
involve all the actions noted above for each file needed.
There is actually a sixth step for most connections—a graceful ending. This is accomplished with another
three-way handshake in which the server sends a packet with the FIN flag set. The client acknowledges
the FIN and the server acknowledges the FIN-ACK.
ACK — refers to the acknowledgment flag of IP packets and to packets in which the ACK flag has been
set. See s and SYN.
client — a computer or application that is making a request. Often used in combination with “server”, the
computer or application that fulfills requests. Both roles are action-based in that the same application or
computer can be a client in one case and a server in a different case.
DNS server — acronym for “Domain Name Service” server. There is actually a hierarchy of DNS servers—
the 13 root servers that know the primary names servers for each of the domains (domains such as edu,
org, com), those actual primary names servers, and local domain name servers. (I also believe there are
additional name servers.) These servers interact with each other and the “whois” database to provide IP
addresses for textual names such as uni.edu.
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ethernet — communication protocol now used mostly on local area networks (LANs). File/message is di-
vided into packets and broadcast to all parties on the network.
FIN — a flag setting in IP packets that indicates communication is complete. A FIN packet will typically
be acknowledged by the recipient in a packet with both the FIN and ACK flags set (FIN+ACK). And, the
acknowledgement will be acknowledged by the originator (an ACK). This process is referred to as a three
-way handshake.
FIN-ACK — refers to the acknowledgment of IP FIN packets in which the FIN and ACK flags have been
set. See FIN.
hub — a device providing connections to a LAN. Computers, printers, etc. are connected to the hub.
When messages are broadcast, all devices on the LAN will “hear” them.
over the Internet from one router to the next, presumably getting closer to the destination with each
transfer. Files being communicated will have been divided into packets and submitted to the IP system
which sends them individually without knowledge or concern for the packet contents or connection to
other packets.
IP address — “internet protocol address”. A number uniquely identifying each machine communicating
on the Internet. The number is typically written in the form ###.###.###.### where each ### is a value
in the range from 1 to 256 (or 0 to 255). IP addresses typically have textual equivalents that are useful for
human consumption, e.g., east.cs.uni.edu is the textual version of
134.161.242.253. The numbers and text components match in reverse order 134-edu, 161-uni, cs-242, east-
253 (this is not precisely correct but it illustrates the point reasonably).
ISP — acronym for “Internet Service Provider”. These companies connect individuals and organizations to
network service providers (NSPs).
LAN — acronym for “local area network”. A set of machines (computers, printers, routers, etc. connected
together. Typically, there is a router included that keeps local traffic local, sends externally bound traffic
on, and accepts inbound traffic addressed to member of the LAN.
nameserver — see DNS server. Also, individual companies are assigned to administer domains such
as .edu, .com, and .org, They assign and keep track of who has which name and provide that infor-
mation to people and servers requesting it.
NAP — acronym for “Network Access Point”. This is merely a connection from a user or organization to an
ISP or for an ISP to an NSP (and, of course, going the other way, an NSP to an ISP and an ISP to an indi-
vidual or organization).
NSP — acronym for “Network Service Provider”. These companies provide the primary infrastructure for
the Internet. If you compare the Internet to the highway system in the US, the NSPs provide the interstate
highway system that have interchanges (NAPs, network access points) for the lesser roads (ISPs) which
connect to city streets (LANs) or (in the country, to) driveways of individuals. Some of the NSPs are AT&T,
MCI, and Sprint.
Packet — a two-part (header and data) collection of information that is the basis for Internet communi-
cation. Breaking large files into pieces ensures that all users have reasonable chance to communicate
and that difficulties in communication are minimized. Many communication protocols use packets. Com-
mon protocols are Ethernet in which packets are broadcast (essentially to the entire LAN or net) and
TCP/IP in which packets are communicated from client to server but with individual packets being sent
over possibly different paths. Packet headers provide information about source and destination, as well
as flag values that affect or control the communication process.
Router — a communication device that receives Internet traffic and passes it along to an appropriate
destination. A router may separate two parts of a network in which case they recognize local traffic and
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keep it local and pass along outbound traffic. In this case, the router will also listen on the non-local side
for traffic addressed to the local net. An alternative placement of the router is just as a node on the net in
which case the router receives traffic, notes its destination, determines the currently best way to forward
the traffic, and forwards it. In both cases the router ensures (to the extent possible) that header infor-
mation was correctly transmitted and that further transmission is appropriate.
server — a computer or application that is fulfilling a request. Often used in combination with “client”, the
computer or application that makes requests. Both roles are action-based in that the same application or
computer can be a client in one case and a server in a different case.
switch — a device providing connections to a LAN. Computers, printers, etc. are connected to the
switch. Messages are arrive at the switch and are directed to the addressee of the message—others on
the LAN do not “hear” them. The switch will pass externally-bound messages to a router and receive in-
bound messages from the outside via the router.
SYN — a flag setting in IP packets that indicates a request to open and synchronize a communication
session. It is typically sent by a client process requesting some service. The SYN will be acknowledged with
a packet which has the ACK flag set and contains a synchronization number (SYN+ACK). The SYN+ACK
will be acknowledged (by the originator) with an ACK packet. This process is referred to as a three-way
handshake.
SYN-ACK — refers to the acknowledgment of IP SYN packets in which the ACK flag has been set and a
synchronization number has been included. See SYN.
TCP — acronym for “transfer control protocol”. It provides the guidelines/mechanisms for sequencing
packets when they are sent over the Internet, checking to see that packets are received and sending
them again when transmission was not successful, and putting the packets back together when received.
Sequence numbers (beginning with a random number but then being incremented by one) provide con-
tinuity in communication. TCP sessions typically begin and end with three-way handshakes.
TCP-IP — the principle Internet communication protocol suite. It indicates and governs how communica-
tion will occur on the Internet. See the separate entries for TCP and IP.
three-way handshake — a set of communications that establishes or severs a TCP connection between a
client and server. The process begins with a SYNchronization request (from a client) or FINalized communi-
cation (from a server), is followed by an annotated acknowledgment (SYN+ACK from server or FIN+ACK
from client), and is terminated by a simple acknowledgment from the originating (client or server). Se-
quence numbers (beginning with a random number but then being incremented by one) provide conti-
nuity in communication.
URL — acronym for “Universal Resource Locator”. It includes the host ID and a path/file name indicating
the name and location of the desired file. An example is www.cs.uni.edu/east/web/index.html. The host
id is “www.cs.uni.edu”, the file is “index.html” and it is located in the “web” subdirectory of the “east” di-
rectory.
whois — a registry of internet domain names. The database identifies an organization and/or person(s)
associated with a particular domain. It works much like a phone book. (Apparently multiple copies of the
database exist and are hosted by different companies/entities.)
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WHAT HAVE YOU LEARNED?
Post-Test
Direction: Write TRUE if the statement is correct and FALSE if the statement is incorrect.
Write your answer on separate sheet of paper.
ENRICHMENT ACTIVITY 1
Direction: On a separate sheet of paper, enumerate the steps on how the internet works.
Steps 1: ______________________________________________________________________
______________________________________________________________________
Steps 2: ______________________________________________________________________
______________________________________________________________________
Steps 3: ______________________________________________________________________
______________________________________________________________________
Steps 4: ______________________________________________________________________
______________________________________________________________________
Steps 5: ______________________________________________________________________
______________________________________________________________________
10
12
What Do You Already Know ? What Have You Learned?
1. B 1. True
2. C 2. True
3. A 3. True
4. D 4. False
5. B 5. True
6. A 6. True
7. D 7. True
8. B 8. False
9. D 9. True
10.D 10 True
ANWER KEYS
REFERENCES
Online Source
http://computer.howstuffworks.com/internet-infrastructure.htm/printable
http://www.securityfocus.com/infocus/1180
http://ocportal.com/site/pg/how_internet_works
http://computer.howstuffworks.com/question525.htm/printable
http://www.answers.com/topic/proxy-server
http://en.wikipedia.org/wiki/Transmission_Control_Protocol
http://vlaurie.com/computers2/Articles/Name.htm
http://vlaurie.com/index.html
http://www.techsupportalert.com/c04100.htm
http://mvolo.com/blogs/serverside/archive/2006/10/16/Where-did-my-IIS7-
server-go_3F00_-Troubleshooting-guide-for-_2200_server-not-found_2200_-
errors.aspx
http://www.strum.co.uk/webbery/intranet.htm http://computer.howstuffworks.com/vpn.htm/
printable http://support.microsoft.com/kb/172983 http://www.netbook.cs.purdue.edu/index.htm
http://www.netbook.cs.purdue.edu/othrpags/page15.htm
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How the internet works
This learning module focuses in the technological explanation on how the
internet works—its processes and equipment. This explains how a website can be ac-
cessed by a device using an internet connection,.