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IP Address

IP Address. Network layer protocols in every host, router Router examines IP address field in all IP datagrams passing through it Analogy Zip codes ~ e.g., 10019. network data link physical. network data link physical. network data link physical. network data link physical.

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IP Address

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  1. IP Address

  2. Network layer protocols in every host, router Router examines IP address field in all IP datagrams passing through it Analogy Zip codes ~ e.g., 10019 network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical Network layer

  3. IP Address • An IP address is a 32-bit sequence of 1s and 0s. • To make the IP address easier to use, the address is usually written as four decimal numbers separated by periods. • This way of writing the address is called the dotted decimal format. 11011111 00000001 00000001 00000001 223 1 1 1

  4. IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router’s typically have multiple interfaces host typically has one interface IP addresses associated with each interface 223.1.1.2 223.1.2.1 223.1.3.27 223.1.3.1 223.1.3.2 223.1.2.2 IP Addressing example network 223.1.1.1 223.1.2.9 223.1.1.4 223.1.1.3 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1

  5. A quick look at Binary and Decimal Number format

  6. Decimal (base 10) • Uses positional representation • Each digit corresponds to a power of 10 based on its position in the number • The powers of 10 increment from 0, 1, 2, etc. as you move right to left 1,234 = 1 * 103 + 2 * 102 + 3 * 101 + 4 * 100

  7. Binary (base 2) • Two digits: 0, 1 • To make the binary numbers more readable, the digits are often put in groups of 4 or 8 1010 = 1 * 23 + 0 * 22 + 1 * 21 + 0 * 20 = 8 + 2 = 10 1100 1001 = 1 * 27 + 1 * 26 + 1 * 23 + 1 * 20 = 128 + 64 + 8 + 1 = 201

  8. Conversion • From binary to decimal • Use positional representation as shown in last slide • From decimal to binary (tricky!) • Keep dividing by 2 • Remainders give the digits, starting from lowest power • Let’s look at some examples… • Now we are ready for IP addressing

  9. IP Address Every IP address has two parts: • Network part • Host part IP addresses are divided into classes A,B and C to define -- large, -- medium, and -- small networks. The Class D address class was created to enable multicasting. Class E addresses reserved for future and research.

  10. IP Address classes

  11. Some special IP addresses • 0.0.0.0 – lowest IP address • Not used for a host connected to the Internet • Used for hosts when they start (boot) • 255.255.255.255 – highest IP address • Not used for a host • Used for broadcasting

  12. A B E IP address assignment:DHCP: Dynamic Host Configuration Protocol 223.1.2.1 DHCP 223.1.1.1 server 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 arriving DHCP client needs address in this network 223.1.1.3 223.1.3.27 223.1.3.2 223.1.3.1 Goal: allow host to dynamically obtain its IP address from network server when it joins network

  13. DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP client-server scenario arriving client DHCP server: 223.1.2.5 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs time DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs IP: 223.1.2.4

  14. Numerical example • A software company has 100 employees. • What would be the ideal class from which the company would choose its network IP to prevent wastage of IP addresses? • How many bits would be assigned for network part and • how many bits would be assigned for host part? • The company suddenly goes through increase in number of employees from 100 to 2040. • What would be the ideal class from which the company would choose its network IP to prevent wastage of IP addresses? • How many bits would be assigned for network part and • how many bits would be assigned for host part? • Solve!

  15. IP addressing: CIDR CIDR:Classless InterDomain Routing • subnet portion of address of arbitrary length • address format: a.b.c.d/x, where x is # bits in subnet portion of address • Back to the previous numerical example? How many address wastage? host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/21

  16. Network Address Translation (NAT)

  17. Home network rest of Internet local network (e.g., home network)

  18. NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers

  19. NAT: Network Address Translation • Advantages: • local network uses just one IP address as far as outside world is concerned: min. IP address wastage • can change addresses of devices in local network without notifying outside world: flexibility • devices inside local net not explicitly addressable, visible by outside world (a security plus).

  20. 3 1 2 4 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 10.0.0.1, 3345 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 NAT: Network Address Translation NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 …… …… 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 3: Reply arrives dest. address: 138.76.29.7, 5001

  21. NAT: Network Address Translation • 16-bit port-number field: • 60,000 simultaneous connections with a single LAN-side address!

  22. NAT traversal problem • client wants to connect to server with address 10.0.0.1 • server address 10.0.0.1 local to LAN (client can’t use it as destination addr) • only one externally visible NATted address: 138.76.29.7 • solution 1: statically configure NAT to forward incoming connection requests at given port to server 10.0.0.1 Client ? 10.0.0.4 138.76.29.7 NAT router

  23. 10.0.0.1 NAT router NAT traversal problem • solution 2: relaying (used in Skype) • NATed client establishes connection to relay • External client connects to relay • relay bridges packets between connections 2. connection to relay initiated by client 1. connection to relay initiated by NATted host 3. relaying established Client 138.76.29.7

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