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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 – 8th Recitation

  • 3rd exercise review

  • Computer networks - Introduction

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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

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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.

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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

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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)

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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).

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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).

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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)

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Higher Layers

  • Application based.

  • Endless variety...

  • Examples : (HTTP/POP3/SMTP/FTP/RPC…)

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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.

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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.

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How data travels

  • Addressing

  • Routing

  • Ports

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  • “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.”

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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.

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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

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Interesting Applications

  • Ipconfig

  • Ping

  • Nslookup

  • Tracert

  • Telnet