CE80NCE80NIntroduction to NetworksIntroduction to Networks
&&The InternetThe Internet
Dr. Chane L. Fullmer
UCSC
Winter 2002
February 5, 2002 CE80N -- Lecture #10 2
General InformationGeneral Information
TA Office…– Trailer #15 is no more– Moved to the ISB– The Interdisciplinary Sciences Building (ISB) is
across McLaughlin Drive and to the southeast of Baskin Engineering. It's about a 5 minute walk from BE. URL below is campus map with ISB shown.http://www.ucsc.edu/general_info/images/map-campusBW-lg.gif
Chapter 20Chapter 20
February 5, 2002 CE80N -- Lecture #10 4
Electronic MailElectronic Mail
The first “Killer App”…Allowed users to communicate via
computer – asynchronouslyModern day GUIs hide original
“command line” operationUses client-server architecture
February 5, 2002 CE80N -- Lecture #10 5
Description Of Description Of FunctionalityFunctionality
Electronic mail systems permit complex communications and interactions.– Send a single message to many recipients– Send a message that includes text, audio,
video, or graphics– Send a message to a user on a network
outside the Internet– Send a message to which a computer program
responds
February 5, 2002 CE80N -- Lecture #10 6
The Best Of All WorldsThe Best Of All Worlds
Networks provide the speed of telephone communication and permanence of postal mail.
– Can transfer small notes or large documents
• Caveat: there are size limitations on email attachments at some sites
– Have become extremely popular
February 5, 2002 CE80N -- Lecture #10 7
Each User Has A Mailbox For E-Each User Has A Mailbox For E-mailmail
Like a post office mailbox, each e-mail mailbox has an address.
Any user can send e-mail to another user if they know the mailbox address.
Only the owner of the box can open the mailbox – but others can snoop the mail while enroute.
February 5, 2002 CE80N -- Lecture #10 8
Sending An E-mail MessageSending An E-mail Message
To send e-mail across the Internet, the user:– Runs an e-mail application– Composes and edits a message
• Adds attachments
– Specifies a recipient– Finishes entering the message– Sends the message
Hey, where’s the send button?
February 5, 2002 CE80N -- Lecture #10 9
EMAIL AttachmentsEMAIL Attachments
Attachments are not added strictly “as-is”– Must be converted to text only characters
for proper operation in mail servers. – Attachments are encoded using a well
known method:• MIME: (Multipurpose Internet Mail Extensions) • BinHex: Macintosh systems• UUENCODE: Unix systems and PCs
February 5, 2002 CE80N -- Lecture #10 10
Notification That E-mail Has Notification That E-mail Has ArrivedArrived
A user can configure the e-mail software to:– Print text messages when mail arrives
• “You’ve got mail”
– Play a recording or tone when mail arrives
• “You’ve got mail!”
– Set an icon on the screen– Suppress notification
altogether
February 5, 2002 CE80N -- Lecture #10 11
Reading An E-Mail MessageReading An E-Mail Message
When the e-mail application begins, it:– Tells the user about waiting messages– Gives an initial summary of the mail– Displays the message contents– Allows the user to:
• Send a reply• Leave the message in the inbox• Save the message• Delete the message
February 5, 2002 CE80N -- Lecture #10 12
E-mail Messages Look Like E-mail Messages Look Like Interoffice MemosInteroffice Memos
An e-mail message begins with a header:
From: To: Date: Subject:
February 5, 2002 CE80N -- Lecture #10 13
E-mail Software Fills In Header E-mail Software Fills In Header InformationInformation
User-friendly software hides unnecessary header lines when
displaying an e-mail message. See the example below:
Received: from amazon.com ([208.33.217.124]) by company1.com with Microsoft SMTPSVC(5.5.1877.447.44); Sat, 3 Jun 2000 12:14:03 -1000Received: by amazon.com id OAA04950; Sat, 3 Jun 2000 14:28:33 -0700 (PDT)Date: Sat, 3 Jun 2000 14:28:33 -0700 (PDT)Message-Id: <[email protected]>X-AMAZON-TRACK: [email protected]: fathers-day-4MIME-Version: 1.0Content-Type: multipart/alternative; boundary=amazonFrom: Amazon.com <[email protected]>Subject: Save $25 at Amazon.com's New Tools & Hardware StoreTo: [email protected]: [email protected]
February 5, 2002 CE80N -- Lecture #10 14
How E-mail WorksHow E-mail Works
E-mail systems follow the client server approach.
– Cooperate to send an email message
• From sender to recipient’s mailbox
• Sender’s computer is the client
– Contacts an e-mail server program on the recipient’s computer
– Stores the message in the recipient’s mailbox
Figure 20.1Figure 20.1
Figure 20.1 An e-mail transfer across the Internet requires two programs: a client on the sender’s computer and a server on the recipient’s computer.
February 5, 2002 CE80N -- Lecture #10 16
Using E-mail From A Personal Using E-mail From A Personal ComputerComputer
Most personal computers do not receive e-mail directly.
– Arrange to have a mailbox on a large computer (I.e, at the ISP)
– Contacts the main computer system
• Mail program: Eudora, Netscape, etc…
– Obtains a copy of their mailbox
February 5, 2002 CE80N -- Lecture #10 17
EMAIL remote operation w/PCsEMAIL remote operation w/PCs
Mail Server A Mail Server B
Sender A
Recipient B
February 5, 2002 CE80N -- Lecture #10 18
Mailbox Address FormatMailbox Address Format
Addresses consist of a string of characters separated by the “@”.
[email protected] The prefix:
– identifies the user.
The suffix:– gives the domain name of the computer on which
the user’s mailbox resides.
February 5, 2002 CE80N -- Lecture #10 19
Abbreviations Make E-mail Abbreviations Make E-mail FriendlyFriendly
Most e-mail systems allow a user to define abbreviations for the mailbox address. For example:
venus.engineering.somecompany.com
Becomes eng
Allowing the address of jane@eng
(Instead of [email protected])
February 5, 2002 CE80N -- Lecture #10 20
Aliases Permit Arbitrary Aliases Permit Arbitrary AbbreviationsAbbreviations
Most commercially available software supports an e-mail alias.
– Requires the user to prepare a list of aliases
– Translates the alias to a longer e-mail address
• ‘mary’ can be used in place of:
February 5, 2002 CE80N -- Lecture #10 21
Aliases Shared By All Users Of Aliases Shared By All Users Of A Computer SystemA Computer System
System-wide aliases make it possible for all users to share abbreviations.– Consider these:
• Webmaster
• Listmanager
• Help
• Printers
February 5, 2002 CE80N -- Lecture #10 22
Sending To Multiple RecipientsSending To Multiple Recipients
E-mail systems allow users to send messages to multiple recipients.– Specify multiple mailbox addresses on the
‘To’ line of message• For example:
February 5, 2002 CE80N -- Lecture #10 23
Mailing List: An Alias for Mailing List: An Alias for Multiple RecipientsMultiple Recipients
A mailing list is an e-mail alias that specifies multiple recipients
The system:– Sends a message to the alias– Delivers a copy to each recipient on the
list
February 5, 2002 CE80N -- Lecture #10 24
Public Mailing Lists And Mail Public Mailing Lists And Mail ExplodersExploders
A public list permits a user on any computer connected to the Internet to send a message to a list of recipients.
When the e-mail message reaches the destination computer, an exploder:
– Finds the name
– Expands the abbreviation
– Forwards a copy to each recipient
Figure 20.2Figure 20.2
Figure 20.2 The path of a mail message sent to a public mailing list that contains three recipients. A Mail exploder retrieves the message, and forwards a copy to each recipient on the list.
February 5, 2002 CE80N -- Lecture #10 26
E-mail To And From Non-E-mail To And From Non-Internet SitesInternet Sites
E-mail can be forwarded to other networks.
For example, Compuserve:– Does not use the same Internet
protocols– Uses software on an intermediate
computer to send and receive Internet e-mail
February 5, 2002 CE80N -- Lecture #10 27
Access To Services Via E-mailAccess To Services Via E-mail
A computer program can:– Be used to answer and reply to an e-
mail message.An e-mail message can:
– Be used to provide access to a variety of remote services.
February 5, 2002 CE80N -- Lecture #10 28
Speed, Reliability, And Speed, Reliability, And ExpectationsExpectations
E-mail systems are more reliable than postal mail systems.– Delivery is usually within minutes
– Sender is notified if a message cannot be delivered
Not all homes have computers connected to the Internet.
Differences in expectations can make e-mail frustrating.– Not all users read their email at the same rate
February 5, 2002 CE80N -- Lecture #10 29
Impact And Significance Of Impact And Significance Of Electronic MailElectronic MailAfter using it, email benefits become
apparent.
– Combines benefits of instantaneous communication with freedom from interruption
– Provides a way for groups to share common interests
– Can communicate with more people
February 5, 2002 CE80N -- Lecture #10 30
Joining A Mailing ListJoining A Mailing List
To join a list, the user must send a request via e-mail.
The request is not sent to the list but to a second alias used for joining or leaving a list.
– To join: [email protected]
– Email to: [email protected]
February 5, 2002 CE80N -- Lecture #10 32
GlossaryGlossary
Electronic Mail – (Email) A service that permits one to send a
memo to another person, a group, or a computer program.
Mail Alias – A synonym for email alias.
February 5, 2002 CE80N -- Lecture #10 33
GlossaryGlossary
Mailbox – A storage area, usually on disk, that holds
incoming e-mail messages until a user reads the mail.
Mailbox Address – A synonym for e-mail address.
February 5, 2002 CE80N -- Lecture #10 34
GlossaryGlossary
Mailing List – An electronic mail address that includes a
list of recipients. Postmaster
– By convention, an e-mail alias for the person who manage the electronic mail software on a given computer.
February 5, 2002 CE80N -- Lecture #10 35
GlossaryGlossary
Smiley – A sequence of characters, usually found in an
e-mail message, that indicates humorous intent. The three character sequence :-) resembles a smiling face turned sideways.
POP – Abbreviation for Post Office Protocol.
Midterm ResultsMidterm Results
February 5, 2002 CE80N -- Lecture #10 37
Midterm ResultsMidterm Results
50 questions– High score: 48 = 96%– Low score: 23 = 46%– Median: 38.4 = 76.8%– Passing grade >= 30
February 5, 2002 CE80N -- Lecture #10 38
Most missed questionsMost missed questions
#11, (77): Prefix bits#38, (73): Routing protocol#30, (60): LAN technologies#19, (56): IP layer#27, (55): Distributed computing#10, (54): CIDR network bits#32, (54): Packet switching
February 5, 2002 CE80N -- Lecture #10 39
Question #11Question #11
11) How many prefix bits (network bits in CIDR) are contained in the Class A address 10.0.0.0
a. 32
b. 24
c. 16
d. 8 Class A has a fixed prefix of 8 bits
e. 10
February 5, 2002 CE80N -- Lecture #10 40
Question #38Question #38
38) The Routing Information Protocol (RIP) only communicates routing information with its directly attached neighbors. RIP is an example of what type of routing protocol?
a. Link state
b. Round-robin
c. Distance vector RIP is distance vector
d. Service oriented
February 5, 2002 CE80N -- Lecture #10 41
Question #30Question #30
30) LAN Technologies, in general are:
a. Not Compatible LANs are not compatible
b. Highly interoperable
c. Proprietary
d. Plug and Play
February 5, 2002 CE80N -- Lecture #10 42
Question #19Question #19
19) IP resides in what layer of the protocol stack (Berkeley or OSI)
a. Routing layer
b. Transport layer
c. Link layer
d. Network layer IP is in the network layer
e. Physical layer
February 5, 2002 CE80N -- Lecture #10 43
Question #27Question #27
27) Any interaction that involves two or more computers over a network is called:
a. Local communication
b. Distributed computing
c. True networking
d. Universal service
February 5, 2002 CE80N -- Lecture #10 44
Question #10Question #10
10) Based on CIDR addressing, how many network bits are contained in the address range 128.114.100.0/20
a. 128
b. 0
c. 100
d. 20 CIDR network bits
February 5, 2002 CE80N -- Lecture #10 45
Question #32Question #32
32) Packet switching is more efficient than circuit switching because of
a. Faster router speeds
b. Resource sharing
c. Short packets are always used
d. Ethernet LANs
e. All of the above
February 5, 2002 CE80N -- Lecture #10 46