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Network Layer

Network Layer. Computer Networks John Ourada. Where are we?. Will Layer 2 Networking Suffice?. Motivation. Connect various link technologies to form a larger internetwork Universal addressing scheme required General purpose use Hides underlying technologies from end user

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Network Layer

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  1. Network Layer Computer Networks John Ourada John Kristoff

  2. Where are we? John Kristoff

  3. Will Layer 2 Networking Suffice? John Kristoff

  4. Motivation • Connect various link technologies to form a larger internetwork • Universal addressing scheme required • General purpose use • Hides underlying technologies from end user • Facilitate communicate between autonomous domains • Able to move packets between any host on the internetwork John Kristoff

  5. Connecting Heterogeneous Networks • Computer System used • Special purpose • Dedicated • Works with LAN or WAN technologies • Known as • router • gateway John Kristoff

  6. Illustration of a Router • Cloud denotes an arbitrary network • One interface per network John Kristoff

  7. Important Idea A router can interconnect networks that use different technologies, including different media and media access techniques, physical addressing schemes or frame formats. John Kristoff

  8. The Internet Concept John Kristoff

  9. Key Functions of the Network Layer • Global Addressing • Fragmentation • Routing We’ll be primarily concerned with addressing and routing John Kristoff

  10. Example Network Layer: Internet Protocol (IP) • Standardized by IETF as RFC 791 • Most popular Layer 3 protocol • Core protocol used on the public Internet • Connectionless protocol • datagrams contain identity of the destination • each datagram sent/handled independently • Of utmost importance for this class! John Kristoff

  11. IP Addressing • Provides an abstraction • Independent of hardware (MAC) addressing • Used by • higher layer protocols • applications John Kristoff

  12. IP Address • Virtual • only understood by software • Used for all communication across an internetwork • 32-bit integer • Unique value for each host/interface John Kristoff

  13. IP Address Assignment An IP address does not identify a specific computer. Instead, each IP address identifies a connection between a computer and a network. A computer with multiple network connections (e.g., a router) must be assigned one IP address for each connection. John Kristoff

  14. IP Address Details • Divided into two parts • prefix identifies the network • suffix identifies the host/interface • Global authority assigns unique prefix for the network • Local administrator assigns unique suffix for the host/interface John Kristoff

  15. Class of IP Addresses (Historical) • Initial bits determined the class • The class determines the boundary between prefix and suffix John Kristoff

  16. Dotted Decimal Notation • Shorthand for IP addresses • Allows humans to avoid binary • Represents each octet in decimal separated by dots • NOT the same as names like www.depaul.edu John Kristoff

  17. Examples of Dotted Decimal Notation • Four decimal values per 32-bit address • Each decimal number • represents eight bits • is between 0 and 255 inclusive John Kristoff

  18. Classes and Network Size (Historical) • Maximum size determined by class of address • Class A large • Class B medium • Class C small John Kristoff

  19. Addressing Example John Kristoff

  20. Illustration of Router Addresses • Address prefix identifies the network • Need one address per router connection John Kristoff

  21. Special Addresses • Network Address not used in packets • Loopback addresses never leave the local computer John Kristoff

  22. IP Addressing: Problems with Classes • Internet growth • Routing table size • Exhaustion of addresses • Administration overhead • Misappropriation of addresses John Kristoff

  23. IP Addressing: Solutions • Subnetting • Variable Length Subnet Mask (VLSM) • Supernetting • Classless InterDomain Routing (CIDR) John Kristoff

  24. Subnetting • Split the suffix into a local network portion and a smaller host id portion • Subnet mask becomes 255.255.255.0 for an 8-bit subnet mask John Kristoff

  25. Subnetting • Subnet boundaries fall between any of the 32 bits in an IP address • Can be complex and confusing, know binary if not not on 8-bit boundaries John Kristoff

  26. John Kristoff

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  29. VLSM • Variable Length Subnet Mask • Can be complex and confusing, know binary! • Use addresses more efficiently. John Kristoff

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  32. Supernetting • Combine multiple smaller address classes into a larger block John Kristoff

  33. CIDR • Classless Inter-domain Routing • Employ supernetting information in IP routers • Advertise smaller CIDR blocks • Decreases the routing table size John Kristoff

  34. IP Packet (datagram) Format John Kristoff

  35. IP Datagrams • Can be delayed • Duplicated • Delivered out of order • Lost • Can change routes from packet to packet • Are connectionless John Kristoff

  36. IP Routing • Performed by routers • Table-driven • Forwarding on a hop-by-hop basis • Destination address used for route determination John Kristoff

  37. Routing/Forwarding Overview • Strip off layer 2 headers/trailers • Extract destination address field, D • Look up D in the routing table • Find next hop address, N • Send datagram to N • Add on layer 2 headers/trailers John Kristoff

  38. Routing Basic Operation John Kristoff

  39. Routing Basic Operation John Kristoff

  40. Basic Routing Operations John Kristoff

  41. Basic Routing Operations John Kristoff

  42. John Kristoff

  43. TCP/IP Routing John Kristoff

  44. TCP/IP Routing John Kristoff

  45. TCP/IP Routing John Kristoff

  46. TCP/IP Routing John Kristoff

  47. ARP Protocol • ARP: Address Resolution Protocol • Resolves IP address to MAC address • Node sends broadcast looking for another node • 140.192.23.1 broadcasts looking for 140.192.23.23 • Node replies with MAC address • 140.192.23.23 replies with 00600A34AA3C • ARP Table: contains records of learned relationships. John Kristoff

  48. Example IP Routing Table • Table (b) is for center router in (a) John Kristoff

  49. Routing Table Size Since each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in the internetwork. John Kristoff

  50. Key Concept The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header. John Kristoff

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