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Chapter 4: Network Layer

4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6. 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP

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Chapter 4: Network Layer

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  1. 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Chapter 4: Network Layer Network Layer

  2. IP Addresses Network Layer

  3. Special IP addresses • The IP address 0.0.0.0 • Is used by hosts when they are booting • IP addresses with 0 as network number • Refer to the current network Network Layer

  4. Class A, B, and C networks: default masks without subnetting • Routers use a default mask • To define size of the network and host parts of address • Default mask • is a 32 bit binary number written in dotted-decimal • defines the structure of an IP address • Identifying the size of the network part of an IP address • Class A mask has a default mask of 255.0.0.0 • Class B default mask => 255.255.0.0 • Class C default mask => 255.255.255.0 Network Layer

  5. A typical campus network A E B F C G D H Network Layer

  6. Subnets • Main objective • Allow networks to be split into several parts (subnets) • For internal use and still act like a single network to outside • Idea • Some bits are taken away from the host number • To create a subnet number • A third part appears in the middle of the address Network Layer

  7. Subnets: example • The main router uses a subnetmask • Indicating the split between • network + subnet and host • The subnet mask in this case is 255.255.252.0 • alternative notation is /22 indicating a 22 bit long mask • Outside the network, subnetting is not visible Network Layer

  8. How IP packets are processed at a router • Without subnetting • Each router has a routing table listing • Some number of network IP addresses • Telling how to get to distant networks • Some number of local host IP addresses • Telling how to get to local hosts • With subnetting • Router table is reduced furthermore • By creating a three-level hierarchy (network, subnet, and host) • A router on subnet k • Knows how to get to all other subnets and to local hosts • does not have to know details about hosts on other subnets Network Layer

  9. Scaling the IP address for the Internet • In the early 1990s • It became apparent that Internet was growing so fast • That all IP addresses would be assigned by mid-1990s • new organizations would be unable to connect to Internet • Several solutions were developed • That allowed the Internet to grow • Without letting us run out of IP addresses • Classless Interdomain Routing (CIDR) • Network Address Translation (NAT) Network Layer

  10. 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 223.1.1.2 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 IP Addressing 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 Network Layer

  11. IP address: subnet part (high order bits) host part (low order bits) What’s a subnet ? device interfaces with same subnet part of IP address can physically reach each other without intervening router Subnets 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 subnet 223.1.3.2 223.1.3.1 network consisting of 3 subnets Network Layer

  12. Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Subnets Subnet mask: /24 Network Layer

  13. How many? Subnets 223.1.1.2 223.1.1.1 223.1.1.4 223.1.1.3 223.1.7.0 223.1.9.2 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer

  14. host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/23 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 Network Layer

  15. Exercises • You have a class C network, and you need to design it for 7 usable subnets with each subnet handling a minimum of 18 hosts each. Which of the following network masks should you use? • 255.255.224.0 • 255.255..255.230 • 255.255.255.224 • 255.255.255.240 • None of the above Network Layer

  16. Exercises • If a host on a network has the address 172.16.210.0/22, what is the address of the subnetwork to which the host belongs? • 172.16.42.0 • 172.16.107.0 • 172.16.208.0 • 172.16.255.208 • 172.16.254.0 Network Layer

  17. 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 223.1.1.2 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 IP Addressing 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 Network Layer

  18. IP address: subnet part (high order bits) host part (low order bits) What’s a subnet ? device interfaces with same subnet part of IP address can physically reach each other without intervening router Subnets 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 subnet 223.1.3.2 223.1.3.1 network consisting of 3 subnets Network Layer

  19. Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Subnets Subnet mask: /24 Network Layer

  20. host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/23 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 Network Layer

  21. IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Network Layer

  22. CDR – Classless InterDomain Routing A set of IP address assignments. 5-59 Network Layer

  23. CIDR (ctd) • Routing tables: • Address Mask • C: 11000010 00011… 255.255.11111000.0 • E: 194.24.00001… 255.255.11111100.0 • O: 194.24.0001… 255.255.11110….. • A packet addressed to 194.24.17.4 • Matches the Oxford base • A router • with a single line for all 3 universities => • Three entries may be combined: 194.24.0.0/19 Network Layer

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