Introduction to Computer
Networks
MGS 602, Fall 2012
What is Networking?
•
Networking involves connecting computers
and other electronic devices for the
purpose of
sharing
information and
resources and for communication
•
A great deal of technology is required for
one device to connect and communicate
with another, and many choices for
physical connections and related software
are possible
Networking Fundamentals
•
An elementary network consists of two computers
connected by some kind of transmission medium
•
Motivation: need to share data and to
communicate quickly and efficiently
–
Sharing enables users to exchange information and
route data between them as workflow demands
–
Can improve human communication substantially
–
Peripheral
device sharing
enables users to take
advantage of peripherals and other devices attached
directly to a network or to a generally available
computer attached to a network
Network Protocols
•
Network protocol:
common set of rules that
allows two computers on a network to
communicate with one another successfully
–
How to interpret signals, how to identify a
computer on a network, how to initiate and end
networked communications, and how to manage
information exchange across the network medium
•
Examples:
–
TCP/IP
–
NetBEUI
–
IPX/SPX
OSI Model
•
A Seven
-
Layer model that describes
functions
for computers to talk to each other.
•
Proposed by ISO in 1984.
•
Each layer can only
talk
to the layer
above/below it.
•
Reference guide for any communication
system.
OSI Model
•
Layering accommodates new technologies
independently.
–
Layer 1 gets Wired to Wireless but upper layers
shouldn’t care.
–
Layer 7 gets email and twitter but Layer 1
shouldn’t care.
•
OSI and other standardization efforts allow us
to mix and match hardware.
–
N
ot restricted by vendor’s proprietary
hw
/sw.
Open Vs. Proprietary Systems
•
Why do closed/proprietary systems exist?
–
Competitive Advantage
–
Collect fees from others
•
Technical openness needed for different
hw
/
sw
talking to each other.
•
Examples of Open
-
Systems
in Networks:
–
TCP/IP, Ethernet.
7 Layers of OSI model
•
Layer 7
–
Email Client / Browser
•
Layer 6
–
File Encryption / Format Conversion
•
Layer 5
–
Session between Web Server and
browser
•
Layer 4
–
HTTP (port 80) / FTP (port 21)
•
Layer 3
–
IP addresses (software) and Routing
•
Layer 2
–
MAC addresses (hardware)
•
Layer 1
–
Electric Signals!
Why care about OSI?
•
Guideline for network
stds
, devices and
internetworking schemes.
•
Breaks operations into less
-
complex elements.
•
Helps engineers focus on modular functions.
•
Standardize interface for plug
-
and
-
play
compatibility.
•
Help multivendor integration.
How Two Computers Communicate
•
TCP/IP is the most common protocol (language)
used on networks
•
TCP/IP uses 2 addresses to identify devices on a
network
–
Logical address (called IP address)
–
Physical address (called MAC address)
•
Just as a mail carrier needs an address to deliver
mail, TCP/IP needs an address in order to deliver
data to the correct device on a network
•
Think of the Logical address as a zip code and the
Physical address as a street address
TCP/IP Physical Layer
•
Physical interface between a
computer
or
terminal
and a transmission medium
•
Specifies:
–
Characteristics of medium
–
Nature of signals
–
Data rate
TCP/IP Internet Layer
•
An Internet is an interconnection of two or
more networks
•
Internet layer handles tasks similar to network
access layer, but between networks rather
than between nodes on a network
•
Uses IP for addressing and routing across
networks
•
Implemented in workstations
and
routers
TCP/IP Transport Layer
•
Also called host
-
to
-
host layer
•
Reliable exchange of data between
applications
•
Uses TCP protocols for transmission
TCP/IP Application Layer
•
Logic needed to support variety of
applications
•
Separate module supports each type of
application (e.g. file transfer)
TCP and UDP
•
Most TCP/IP applications use TCP for transport
layer
•
TCP provides a connection (logical association)
between two entities to regulate flow check
errors
•
UDP (User Datagram Protocol) does not
maintain a connection, and therefore does not
guarantee delivery, preserve sequences, or
protect against duplication
TCP/IP Applications
•
SMTP (Simple Mail Transfer Protocol)
–
Basic e
-
mail facility, transferring messages among hosts
•
FTP (File Transfer Protocol)
–
Sends files from one system to another on user command
•
Telnet
–
Remote login capability, allowing a user to emulate a
terminal on the remote system
TCP Segment and Port
•
Source
port
(16 bits)
•
Destination
port
(16 bits)
•
Sequence number (32 bits)
•
Acknowledgment number
(32 bits)
•
Data Offset (4 bits
)
•
Flags (6 bits) : URG, ACK, PSH, RST, SYN,
FIN
In Ubuntu, use
netstat
-
ln
–
tcp
to see
open ports
(those that are accepting packets)
TCP Segment
IP Address
•
IP provides for 32
-
bit source and destination addresses
•
IPv6 (1996 standard) provides for 128
-
bit addresses
•
Migration
to IPv6 will be a very slow
process
•
IP Header:
–
Time to Live (8 bits
)
–
Type of Service (8 bits
)
–
Fragment Offset (13 bits
)
In Ubuntu, use
ifconfig
and then look for
inet
addr
(IP addresses associated with the local network interfaces)
IP Packet
MAC / Physical Address
•
MAC
–
Media Access Control
•
Unique in the world for each physical network
card/interface
–
Network Interface Card (NIC)
•
Numbering assigned by IEEE organizations
•
First 6 characters (highest 3 bytes) => Vendor
–
Organizationally Unique Identifier (OUI)
In Ubuntu,
, use
ifconfig
and then look for
HWaddr
(MAC addresses of the
local network interfaces
)
Communication Between Two
Computers
1.
A user at Comp A types ping 10.1.1.2 at a
command prompt
2.
The network software creates a ping
message
3.
The network protocol packages the
message by adding IP address of sending
and destination computers and acquires the
destination computer
’
s MAC address
4.
The network interface software adds MAC
addresses of sending and destination
computers and sends the message
5.
Comp B receives message, verifies that the
addresses are correct and then sends a
reply to Comp A using Steps 2
–
4
33
Local and Wide Area Networks
•
Local Area Network (LAN):
small network, limited
to a single collection of machines and one or more
cables and other peripheral equipment
•
Internetwork:
networked collection of LANs tied
together by devices such as routers
–
The
Internet
is the best example
•
Wide Area Network (WAN):
internetwork that
spans distances measured in miles and links two
or more separate LANs
•
Metropolitan Area Network (MAN):
uses WAN
technologies to interconnect LANs in a specific
geographic region, such as a county or a city
Local and Wide Area Networks
LAN
W
AN
Network Servers
•
Most common server roles found on
networks:
–
Domain controller/directory servers
–
File and print servers
–
Application servers
–
Communication servers
–
E
-
mail/fax servers
–
Web servers
Network Servers
•
Domain Controller/Directory Servers
–
Directory services make it possible for users to locate, store,
and secure information about a network and its resources.
–
Windows servers permit combining computers, users, groups,
and resources into domains. The server handling the computers
and users in a domain is called a domain controller.
•
File and Print Servers
–
Provide secure centralized file storage and sharing and access
to networked printers.
–
Any Windows or Linux computer can act as a file and print
server, however the Server version of Windows provides
advanced sharing features.
Network Servers
•
Application Servers
–
Supply the server side of client/server applications to
network clients
–
Differ from basic file and print servers by providing
processing services as well as handling requests for file or
print services
•
Communication Servers
–
Provide a mechanism for users to access a network
’
s
resources remotely
–
Enable users who are traveling or working at home to dial
in to the network via a modem or their existing Internet
connection
•
E
-
mail/Fax Servers
Network Servers
•
Web Servers
–
Windows Server includes a complete Web server called Internet Information
Services (IIS) as well as File Transfer Protocol (FTP)
–
Apache Web Server is available as a part of most Linux distributions and
remains the most widely used Web server in the world
•
Other Network Services
–
Most networks require additional support services to function efficiently. The
most common are Domain Name System (DNS) and Dynamic Host
Configuration Protocol (DHCP)
–
DNS allows users to access both local and Internet servers by name rather
than by address
–
DHCP provides automatic addressing for network clients so that network
administrators do not have to assign addresses
manually
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