Transcript Lecture04_Net2x

Lecture 4
Networking Part 2
More on WANs
• Fiber Optic Cables
– Used in Internet backbone
– Speeds over 100 Gbps
– Team at the University of Southampton
achieved a throughput of ~70 Tbps, with the
signal traveling at 99.7% the speed of light
(New Scientist)
Network Tools
• Available on site like http://ping.eu/
• Tools
– ping (try millersville.edu)
– DNS lookup
• Visit site using IP address
• Reverse lookup
– traceroute
– whois (authoritative name servers)
Ethernet
• Uses a wire or fiber, called the channel
• Engineers “tap” into the channel to
connect a computer
– Can then send/recv signals
– All computers, including the sender,
can detect the signal
3-5
Ethernet Network Protocol
• Rough analogy: conversation at a cocktail party
– One person talks, everyone listens
– When talker stops, it is briefly quiet, and someone else starts
– If two persons start talking, they hear the conflict and pause
briefly, until one begins talking again
• Decentralized scheme (no plan or schedule control)
– Each computer listens to the channel, and if it's quiet, it's free.
– A computer starts to transmit; if another starts at the same time,
they hear garbled message; both stop for a random time and
then try again.
– Shortest random wait gets the channel
3-6
Ethernet vs. Internet
• Internet
– Uses point-to-point network to implement pointto-point communications
– Allows multiple communications taking place
concurrently along its different paths
• Ethernet
– Uses broadcast network to implement point-topoint communications
– Allows only one communication at a time
3-7
Connecting to the Internet
• Two basic methods
1. Connect via an Internet service provider
(ISP)
2. Connect through a campus or enterprise
network
1. Connections by ISP
• Most home users use ISPs
• Modems convert digital output to carrier
format
1. Connections by ISP
• Signals are sent to carrier
• Carrier modem receives signals, sends to
server
• Server connects to the Internet via a
gateway
• Smart phones also have modems
2. Enterprise Network Connections
• Connect via organization’s network
• Organization connects to the Internet by a
gateway
Wireless Networks
• Variation of a LAN connection
• Protocol name is 802.11
– g@54 Mbps, n@150 Mbps, ac@867 Mbps
• Uses access point/router
– Physically connected to an ISP’s modem
– Capable of broadcasting and receiving
signals, usually radio frequency (rf ) signals
The World Wide Web
• Some computers connected to the Internet
are Web servers
– Supply files to browsers
• Web servers and their files comprise the
World Wide Web (WWW)
The World Wide Web
• Web pages are files
– Text
– Images
– Video
– Audio
– Programs
Requesting a Web Page
• Web requests use client/server interaction
– Browser is client
– Web server is server
– File specified using URL (Universal Resource
Locator)
• Web browsers and servers use HTTP
protocol
Requesting a Web Page
http://cs.millersville.edu/~zoppetti/101/index.html
• The URL has three main parts:
– Protocol
tells the computers how to handle the file
– Server computer’s name
or the name given by the domain hierarchy
– Page’s pathname
tells the server which file (page) is requested
and where to find it
Describing a Web Page
• Servers do not store Web pages as
images
• Pages are stored in source files as a
description of how they should appear on
the screen
• Browser receives the description and
renders image
Describing a Web Page
• Two advantages to storing and sending
the source file
1. Less space
2. Browser can adapt image for your machine
The Internet and the Web
• Some Web servers
– Have www as part of their domain name
– Add www if you leave it out
– Work either way
• When is the www required and when is it
optional?
The Internet and the Web
• Web addresses consist of host names
– And host names have an IP address
• DNS server requires name to match
exactly
The Internet and the Web
• Incorrect name results in
– Accessing wrong IP address
• google.com vs google.gov
– DNS lookup fails (“Server Not Found”)
• Web administrators may register similar
forms of a URL
– yahoo.com
File Structure
• Directory Hierarchy
– Folders can contain folders as well as files
• Think of any hierarchy as a tree
– folders are the branch points
– files are the leaves
File Structure
• Directory Hierarchy
– All hierarchies have branch points and leaves
– Hierarchy trees are often drawn sideways or
upside down
– Two terms are standard, however:
1. Down in the hierarchy means into subfolders
(towards the leaves)
2. Up in the hierarchy means into folders (toward
the root)
File Structure
• Part of the directory hierarchy is shown in
the pathnames of URLs:
airandspace.si.edu/exhibitions/gal100/pioneer.html
• Page is specified by a pathname that
specifies file
– With each slash (/) we move into a subfolder
or to the file
Organizing the Folder
• Last item in pathname is file
– Unless URL ends in /, then server looks for
“index.html”
• Course page example
Summary
• Point-to-point, multicast, broadcast,
synchronous, and asynchronous
communications
• IP addresses, domains, IP packets, IP
protocol, WANS and LANS, Ethernet
protocol, ISPs, enterprise networks, and
wireless networks
Summary
• Difference between the Internet and the
Web
• File hierarchies
Quiz
• How many bytes are in an IPv6 address?
• The ______ translates from host names to
IP addresses.
• TCP/IP is a key ______ used on the
Internet.