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ISP – 8 th Recitation. 3 rd exercise review Computer networks - Introduction. 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”.

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isp 8 th recitation
ISP – 8th Recitation
  • 3rd exercise review
  • Computer networks - Introduction
what is it good for
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

how does it work
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 it’s sometimes hard to distinguish between layers.
basic layers
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
physical layer
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)
datalink layer
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).
network layer
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).
transport layer
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)
higher layers
Higher Layers
  • Application based.
  • Endless variety...
  • Examples : (HTTP/POP3/SMTP/FTP/RPC…)
relevant concepts
Relevant Concepts
  • Most programmers don’t 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.
relevant concepts12
Relevant Concepts
  • IP Address - 32 bits long, unique to each internet node.
  • Subnet mask – 32 bit long. Tells which part of the IP address designate the same “physical” network.
  • Router/Gateway – A computer that’s 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.
how data travels
How data travels
  • Addressing
  • Routing
  • Ports

bu bu but
  • “I’ve been using the internet for decades and never ever set my computer’s IP address or subnet mask, never seen a router/default gateway address and never ever contacted a web page using 4 (darn) numbers.”
dynamic host configuration protocol dhcp
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.
domain name server dns
Domain Name Server (DNS)
  • It’s hard to remember numbers so we don’t normally use IP addresses…
  • DNS servers are catalogs which map IP addresses to names that are easier to instead of
  • The internet contains a hierarchy of DNS servers, each responsible for different names spaces. (.com, .il,,,, . – root )
  • Addresses and name spaces are globally controlled by ICANN / IANA
interesting applications
Interesting Applications
  • Ipconfig
  • Ping
  • Nslookup
  • Tracert
  • Telnet