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Part 4 : Network Layer

Part 4 : Network Layer. Role and Position of Network Layer. Network layer in the Internet model is responsible for carrying a packet from one computer to another It is responsible for host-to-host delivery. Position of network layer. Duties of Network Layer.

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Part 4 : Network Layer

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  1. Part 4 :Network Layer

  2. Role and Position of Network Layer • Network layer in the Internet model is responsible for carrying a packet from one computer to another • It is responsible for host-to-host delivery. • Position of network layer

  3. Duties of Network Layer

  4. Chapter 19Host-to-host Delivery :Interworking, Addressing, and Routing

  5. 19.1 Internetworks • The physical and data link layers of a network operate locally

  6. Links in an Internetwork

  7. Network Layer in an Internetwork

  8. Network Layer at the Source

  9. Network Layer at a Router

  10. Network Layer at the Destination

  11. Switching • Virtual circuit approach – relationship between all packets belonging to a message is preserved – a single route is chosen, and all packets take that route • Datagram approach – each packet is treated independently of all others – thus, packets in the same message can take different routes, and possibly arrive out of order

  12. Datagram Approach

  13. Internet as a Connectionless Network • In a connection-oriented service, the source first makes connection with the destination before sending a packet. • They are sent on the same path in sequential order. • In a connectionless service, the network layer protocol treats each packet independently, with each packet having no relationship to any other packet.

  14. 19.2 Addressing • For a host to communicate with any other host • Need a universal identification system • Need to name each host • Internet address or IP address is a 32-bit address that uniquely defines a host or a router on the internet • The IP addresses are unique in the sense that two devices can never have the same address. However, a device can have more one address.

  15. Notation • Binary notation 01110101 10010101 00011101 11101010 32 bit address, or a 4 octet address or a 4-byte address • Decimal point notation

  16. Notation (cont’d) • Hexadecimal Notation - 8 hexadecimal digits - Used in network programming 0111 0101 1001 0101 0001 1101 1110 1010 75 95 1D EA 0x75951DEA

  17. Classful Addressing • Occupation of address space • In classful addressing, the address space is divided into five classes: A, B, C, D, and E. • Finding the class in binary notation

  18. Classful Addressing (cont’d) • Finding the address class

  19. Classful Addressing (cont’d) • Finding the class in decimal notation

  20. Example 4 • Find the class of each address: a. 227.12.14.87 b. 252.5.15.111 • 134.11.78.56 • Solution a.The first byte is 227 (between 224 and 239); the class is D. b. The first byte is 252 (between 240 and 255); the class is E. c. The first byte is 134 (between 128 and 191); the class is B.

  21. Netid and Hostid • Each IP address is made of two parts; netid and hostid. • Netid defines a network; hostid identifies a host on that network.

  22. Netid and Hostid (cont’d) • IP addresses are divided into five different classes: A, B, C, D, and E

  23. Classes and Blocks • Blocks in class A • Class A is divided into 128 blocks with each block having a different netid. • Millions of class A addresses are wasted.

  24. Classes and Blocks (cont’d) • Class B is divided into 16,384 blocks with each block having a different netid Many class B addresses are wasted.

  25. Classes and Blocks (cont’d) • Class C is divided into 2,097,152 blocks with each block having a different netid. The number of addresses in a class C block is smaller than the needs of most organizations

  26. Classes and Blocks (cont’d) • Class D addresses are used for multicasting; there is only one block in this class. • Class E addresses are reserved for special purposes; most of the block is wasted.

  27. Network Address • The network address is the first address. • The network address defines the network to the rest of the Internet. • Given the network address, we can find the class of the address, the block, and the range of the addresses in the block • In classful addressing, the network address (the first address in the block) is the one that is assigned to the organization.

  28. Network Address (cont’d) • Network address : an address with the hostid all set to 0s

  29. A Sample Internet with Classful Address • Token Ring LAN (Class C), Ethernet LAN (Class B), Ethernet LAN (Class A) , Point-to-point WAN, A Switched WAN

  30. Subnetting and Supernetting • Subnetting • A network is divided into several smaller networks with each subnetwork (or subnet) having its subnetwork address • Supernetting • Combining several class C addresses to create a larger range of addresses • IP Addresses are designed with two levels of hierarchy

  31. Subnetting • Classes A, B, C in IP addressing are designed with two levels of hierarchy (not subnetted) • Netid and Hostid

  32. Subnetting (cont’d) • Further division of a network into smaller networks called subnetworks • R1 differentiating subnets

  33. Subnetting (cont’d) • Three levels of hierarchy : netid, subnetid, and hostid

  34. Subnetting (cont’d) • Three steps of the routing for an IP datagram • Delivery to the site, delivery to the subnetwork, and delivery to the host • Hierarchy concept in a telephone number 031

  35. Default Masks • When a router receives a packet, it needs to route it • Uses mask to determine the subnetwork address • Routers outside the organization use default mask • Routers inside use a subnet mask

  36. Comparison of a default mask and a subnet mask • Number of subnets is determined by number of extra 1s in the subnet mask. • 2n = 23 = 8 subnets

  37. Supernetting • A block of class x addresses • For example, • An organization that needs 1,000 addresses can be granted four class C addresses

  38. Supernetting (cont’d) • 4 class C addresses combine to make one supernetwork

  39. 19.3 Routing • Next-hop routing

  40. Routing (cont’d) • Network-specific routing • Don’t have an entry for every host connected to the same • physical network • Instead, only have one entry to define the destination network

  41. Routing (cont’d) • Host-specific routing

  42. Routing (cont’d) • Default routing

  43. Static and Dynamic Routing Tables • Static routing table : containing information entered manually • Dynamic routing table • updating periodically using one of the dynamic routing protocols such as RIP, OSPF, or BGP • Whenever there is a change in the Internet, the dynamic routing protocols update all the tables in the routers.

  44. 20.2 IP datagram

  45. IP Datagram (cont’d) • Version : for IP version4, it is 4 • Header Length : Defining the length of the datagram header in 4 byte words

  46. IP Datagram (cont’d) • Differentiated Services • The first 6 bits : codepoint subfield (DSCP : differentiated services code point) • Values for codepoints

  47. IP Datagram (cont’d) • Total Length : head + data • Defining the total length of the datagram including the header • Length of data = total length – header length • Limited to 65,535 (216 – 1) bytes • Encapsulation of a small datagram in an Ethernet Frame

  48. IP Datagram (cont’d) • Fields related to fragmentation • Identification : 16 bit-field • Datagram id that is originated by the source host • Therefore, Source IP address + datagram id (identification) • All fragments having same identification number • Identification No. to be used for the destination in reassembling the datagram • Flags : 3 bit-field • D : Do not fragment (1) • If it can not pass the datagram through any available physical network, it discards the datagram and send ICMP error message to the source host • M : More fragment (0) • 0 : last fragment or only fragment

  49. IP Datagram (cont’d) • Fragmentation offset : 13-bit field • Showing relative position of this fragment with respect to the whole datagram • Measured in units of 8 bytes : forcing hosts or routers that fragment datagrams to choose the size of each fragment so that the first byte number is divisible by eight

  50. IP Datagram (cont’d) • Time to live • Used to control the maximum number of hops (routers) visited by the datagram • If the value is Zero, the routers discarded • If the source wants to confine the packet to the local network, it can store 1 in this field

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