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How the Internet Works?
( TCP/IP, DNS, HKIX … )
How computers send data?
Protocol
Connection
method
Channel
Address
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Outline
• Internet Hardware
– Modem, Wired, Hubs, Switches
• Basic Internet Structure
– Postal service analogy
• Internet Software
– IP address, Subnets
– Network Protocol
– DNS, WINS and Domains
• HKIX and HARNET
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Communication Channel
what kind of media?
• Telephone line (Twist pair, Optical fiber)
– Modulator-Demodulator (Modem)
– Digital Subscriber Line (DSL)
• Cable modem
• Satellite, Microwave
• Wireless connection (IR, RF)
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Different medium
•
•
•
•
•
•
Twisted pair
RJ45 connector
Coaxial cable
BNC connector
Fiber optic cable
Different cabling length
– Optic > coaxial > Twisted pair
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Use Modem
Digital data - 10101010101
Telephone line
Analog data
• Modulation / De-modulation
• E.g. CU Dialup Pool (56kbps max.)
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Use Digital Subscriber Line (DSL)
Traditional phone line
ADSL modem
Why ADSL?
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Why ADSL?
• Asymmetric DSL
– A slower upstream (upload)
can trade off a faster
downstream (download)
speed.
– 128 to 640 kbps (upstream)
– 1.5M to 5M bps
(downstream)
• E.g. PCCW - Netvigator
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Use Cable Modems
• Use coaxial cable to carry TV signal and High
speed Internet access
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Satellite &
Microwave
• Satellite systems
– Use satellites orbiting above the Earth to relay
signals from one part of a WAN to another, cause
0.5 to 5 sec delays.
• Microwave
– Costly to install but cheaper than satellite.
– It is very useful for connecting networks that are
separated by a barrier, such as a highway or a lake.
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Use Cellular Phone
• Mobile computing
• Latest technology – Bluetooth
– A wireless technology
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Using A Computer To Interconnect
Networks
• Special-purpose
computers are used to
interconnect networks.
– Using standard hardware
(CPU, memory, and network
interfaces)
– Running special-purpose
software
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Network Interface Card
NIC
• Physically connects a computer to the transmission
medium on a network.
• What is device driver?
– NIC comes with different drivers for different types of
operating systems.
– A driver is the software that allows the operating system to
communicate with the network interface card.
• What is the major difference between an expensive,
3Com, and a cheap, Filand, NIC?
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Routers
• Interconnecting computers are called routers
by using the same protocol.
– Determining where to send packets
Router
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Hubs
• A network cable connects a
computer via a network card to
a hub.
• Provides a central location.
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Ports
• A hub
contains
sockets or
ports.
• Some LED
indicates
information
transferred
through the
port.
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Ethernet Repeater
• A repeater is a device that strengthens
and retransmits signals on a network.
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Network Architecture
• It refers how information transfers on networks.
• Ethernet
– It the most popular architecture used to build networks.
– Least expensive and easiest to setup
• Token-ring architecture
– It was developed by IBM in 1984.
– They are popular found in large organizations, such as
banks and insurance companies.
• Others: ARCnet, AppleTalk, …
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Ethernet & Token Ring
• Ethernet, Fast Ethernet, Gigabit Ethernet
(transmit data at 1Gbps).
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Packet switching example
Figure 16.1 An example
internet with four networks
connected by routers.
Figure 16.2 Cars from two roads merging
onto another road are analogous to packets
from two networks merging onto a third
network.
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IP address
• Each host in the Internet is assigned to a
specific and unique number for
identification.
• This number is called the IP address of
the specific host.
• This number is divided into 4 parts for
improving the readability.
• The range of each number is between 0
and 255.
– E.g. 0.0.0.0
– 255.255.255.255
• For example, the host
“orchid.cse.cuhk.edu.hk” has its IP
address of “137.189.91.60”
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Network Number /
Host Number
• IP addresses are split into 2 parts
– A network number + a host number
• For example, 137.189 is the network number of
CUHK, 91.60 is the host number of the host
“orchid.cse.cuhk.edu.hk”
• Network numbers are assigned by a central authority,
the Internet Corporation for Assigned Names and
Numbers (ICANN).
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Network Classes
• There are 5 classes of IP address
• Class A comprises networks 1.0.0.0 to
127.255.255.255, the network address is in first quad.
It allows roughly 16 million hosts per network.
• Class B comprises network 128.0.0.0 to
191.255.255.255, the network address is in the first
two quads. It allows for 16,382 networks with up to
64K hosts.
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Network Classes
• Class C comprises networks 192.0.0.0 to
223.255.255.255, with the network number
contained in the first three quads. It allows
about 2 million networks with up to 254 hosts
in each network.
• Class D and E are falling into the range of
224.0.0.0 to 254.0.0.0 which are reserved for
multicast address and for special purpose use.
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Subnet
• The Internet is structured hierarchically. CUHK consists of
many academic departments and administrative bodies.
• IP allows you to subdivide a network into several subnets. E.g.
CSE and CSC are two subnets inside CUHK.
• Each subnet is identified by a subnet number.
• E.g. we have a different way to interpret the IP address
137.189.91.60
– 137.189 refers to the network number of CUHK,
– 91 refers to the subnet number of CSE,
– 60 refers to the host number of “orchid”.
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Subnet Mask
• 252  254 = 1111 1100  1111 1110
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Dynamic Host Configuration Protocol
(DHCP)
• A Protocol for assigning dynamic IP address to
devices on a network.
• It is built on client and server models.
– Server is the machine running DHCPD.
– Client can be any network devices.

Advantage?
Eliminates manual
configuration of network
parameters and utilizes the use
of IP address
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IP Routing
• The process of transmitting a data packet from the
source to the destination via a series of
intermediate stations is called “Routing”.
• IP routing works as follow:
– Each data packet is labeled with IP address of the
destination host
137.189.90.184 1500 bytes Data here
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IP Packet
• Includes Header, payload, data
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Packets Are Not The Same Size
• Packets may be any size up to the maximum.
– Can be as small as a single keystroke
– Can be larger, depending on application
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Protocol and Addressing
TCP/IP model
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Protocol and Addressing
• Internet has a large collections of protocols organized
in a layering model.
– Application: enables the user, whether human or software,
to access the network.
– Transport: responsible for source-to-destination (end-toend) data transfer.
– Network: responsible for routing packets from source-todest across multiple networks.
– Data link: responsible for data transfer between
neighboring network elements.
– Physical: coordinates the functions required to transmit a
bit stream over a physical medium.
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Protocol and Addressing

Lower layer adds header to the data from upper layer.
Header includes addressing and other fields.
L5 data
L5 data
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Protocol Stack - Open Systems Interconnection
(OSI) model
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Transmission Control Protocol - TCP
• Basic functions
– Decompose a lengthy data into multiple packets
for transmission
– Error detection, ensure validity
– Packet loss?
• No problem, packet retransmission
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Packet Retransmission
• A packet may be lost during the
transmission across the Internet (host down,
link failure, … )
• When the destination host has been waiting
for a particular packet for a certain time
(timeout), it will request the source host to
retransmit the packet.
• There is no need to retransmit all data
packets. Instead, only the missing packet,
which is identified by the sequence number,
needs to be retransmitted.
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Error detection - Checksums
• Transmission errors occur even if a data packet is
received by the destination successfully.
• How to ensure the data is correctly received?
– A method to detect possible transmission errors.
– At the destination, checksum is recalculated based on the
received data.
– The attached checksum and the newly calculated
checksum are compared. Mismatch means there is
transmission errors occurred.
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IPv6 (IP version 6)
• Major changes:
– More addresses
• IP address size from
32 bits to 128 bits
– Simplified IP
headers
• Reduction of header
fields in IP packet
– Added security
features
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Why DNS?
• What is DNS?
– Domain Name System
• IP address is difficult to remember.
– 137.189.92.1 is which machine?
• Names are given to each computer on the Internet
for the convenience of human users.
• Besides IP addresses, all internet applications allow
users to use computer names.
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Computer names on the NET
• How does it look like?
– DNS administrators is responsible to name
computers/group in their own subnet.
– Each level of responsibility is called a domain.
• Domains are separated by “dots”
– cse.cuhk.edu.hk
– www.intel.com
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Domain name and Host name
• Each domain can create or change whatever
belongs to it.
– CUHK can create any new domain, cse
– CSE can buy a new computer and name it as –
robin, orchid, any other name.
• Two computers may have the same name if
they are in different domains.
– orchid.cse.cuhk.edu.hk
– orchid.ie.cuhk.edu.hk
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Domain Name System (DNS)
• Each node in the
tree has
– a label - a string with
a maximum of 63
characters.
– a domain name - a
sequence of labels
separated by dots.
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How does DNS work?
• DNS server = DNS service?
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IP vs Domain Name
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Most Common hostname
Figure 18.1 The fifty most common names assigned to computers on the Internet in 2000.
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References
• Computer Networks – A. S. Tanenbaum
(Prentice Hall)
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