ISP

1
ISP 8th Recitation

• 3rd exercise review
• Computer networks - Introduction

2
What is it good for?

• Communicating and sharing resources around the
  world.
• Common applications
• Web / FTP / Data Transfer
• E-Mail / IM / VOIP / Human Communication
• Internet

In general, a collection of bytes (packet) is
sent from one computer to another
3
How does it work

• Computer networking is based on layers of
  different protocols.
• Layer Additional bytes in the beginning/(end)
  of a message.
• The holy grail of computer networking is the 7
  layers model of OSI (half obsolete)
• In reality, fewer layers are being used and its
  sometimes hard to distinguish between layers.

4
Basic Layers

• Physical layer Signals and cables
• Data-link layer Local communication
• Network layer End to end communication
• Transport layer Reliability/Flow control
• Higher layers Application based

5
Packet Structure
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Physical Layer

• How to physically connect the computer to the
  network (cables/connectors)
• Signals running through the cables
  (voltages/encodings)
• Examples RS232/10BASE-T/802.11(x)

7
Datalink Layer

• How two adjacent computers talk to one another.
• Tied closely to the physical layer.
• Detection and correction of errors.
• Examples Ethernet, Token-Ring, 802.11(x).

8
Network Layer

• Allows transfer of data between far away
  computers.
• Deals with addressing and routing.
• Unreliable - data can get lost and come in the
  wrong order.
• Examples IP (very few others).

9
Transport Layer

• Provides a transparent mean for sending data.
• Can provide reliability, order and flow control.
• Provides virtual ports to set apart different
  data.
• Often, closely tied to the network layer (TCP/IP)
• Examples TCP/UDP (and more)

10
Higher Layers

• Application based.
• Endless variety...
• Examples (HTTP/POP3/SMTP/FTP/RPC)

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Relevant Concepts

• Most programmers dont care much about the
  physical and data-link layers - Too down in
  hierarchy and transparent otherwise.
• Network and transport layers are relevant and
  important to understand.
• Higher layers are program specific
• Interface an existing program using its protocol.
• Create a new program using a self made protocol.

12
Relevant Concepts

• IP Address - 32 bits long, unique to each
  internet node. 192.168.10.11
• Subnet mask 32 bit long. Tells which part of
  the IP address designate the same physical
  network. 255.255.255.0
• Router/Gateway A computer thats connected to
  two or more physical networks and moves data
  between the two based.
• Port a 16bit long number which helps map
  network data to specific applications.

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How data travels

• Addressing
• Routing
• Ports

192.168.1.20 255.255.255.0
192.168.0.1 255.255.255.0
192.168.0.11 255.255.255.0
192.168.5.1 255.255.255.0
192.168.0.15 255.255.255.0
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Bu..Bu..But

• Ive been using the internet for decades and
  never ever set my computers IP address or subnet
  mask, never seen a router/default gateway address
  and never ever contacted a web page using 4
  (darn) numbers.

15
Dynamic Host Configuration Protocol (DHCP)

• As we just saw, each network interface requires
  some network specific IP settings like a unique
  IP address, subnet mask, default gateway and DNS
  servers.
• DHCP servers can sit at each local network and
  simplify the configuration process by allocating
  each network node with all its relevant IP
  settings.
• DHCP servers allow us to easily connect to
  wireless hotspots and ISPs.

16
Domain Name Server (DNS)

• Its hard to remember numbers so we dont
  normally use IP addresses
• DNS servers are catalogs which map IP addresses
  to names that are easier to remember.www.google.c
  om instead of 74.125.53.100.
• The internet contains a hierarchy of DNS servers,
  each responsible for different names spaces.
  (.com, .il, .ac.il, google.com, tau.ac.il, .
  root )
• Addresses and name spaces are globally controlled
  by ICANN / IANA

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Interesting Applications

• Ipconfig
• Ping
• Nslookup
• Tracert
• Telnet