CE80N Introduction to Networks & The Internet

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

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Chapter 20Chapter 20


Electronic MailElectronic Mail

The first “Killer App”…Allowed users to communicate via

computer – asynchronouslyModern day GUIs hide original

“command line” operationUses client-server architecture


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


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


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.


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?


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


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


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


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:


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]


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.


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


EMAIL remote operation w/PCsEMAIL remote operation w/PCs

Mail Server A Mail Server B

Sender A

Recipient B


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.


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])

mailto:jane@eng


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:

– [email protected]


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


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:

– To: [email protected], [email protected]

mailto:[email protected]



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


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.


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


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.


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


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


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]




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.


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.


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.


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


Midterm ResultsMidterm Results

50 questions– High score: 48 = 96%– Low score: 23 = 46%– Median: 38.4 = 76.8%– Passing grade >= 30


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


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



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



30) LAN Technologies, in general are:

a. Not Compatible LANs are not compatible

b. Highly interoperable

c. Proprietary

d. Plug and Play



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



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



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



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


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CE80N Introduction to Networks & The Internet