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ISP – 8 th Recitation

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

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  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 it’s 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

  6. 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…)

  11. 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.

  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 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.

  13. 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

  14. 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.”

  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) • 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 remember.www.google.com 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

  17. Interesting Applications • Ipconfig • Ping • Nslookup • Tracert • Telnet

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