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15 3

  1. 1. Objectives Understanding how a “heterogeneous” collection of networks can be connected to create an internetwork Understanding how to use the hierarchy of the IP address to make routing decisions and make routing in an internet more scalable
  2. 2. Internet Protocol (IP) Part of TCP/IP Used by the Internet Specifies interface with higher layer e.g. TCP Specifies protocol format and mechanisms
  3. 3. IP Services Primitives Functions to be performed Form of primitive implementation dependent ⌧e.g. subroutine call Send ⌧Request transmission of data unit Deliver ⌧Used by IP to notify user of arrival of data unit Parameters Used to pass data and control info
  4. 4. Parameters (1) Source address Destination address Protocol Recipient e.g. TCP Type of Service Specify treatment of data unit during transmission through networks Identification Source, destination address and user protocol Uniquely identifies PDU Needed for re-assembly and error reporting Send only
  5. 5. Parameters (2) Don’t fragment indicator Can IP fragment data If not, may not be possible to deliver Send only Time to live Send only Data length Option data User data
  6. 6. Type of Service Precedence 8 levels Reliability for damage and loss Normal or high Delay Normal or low Throughput Normal or high
  7. 7. IP Protocol
  8. 8. IP Addresses To achieve scalability: Reduce amount of info stored in each node Reduce info exchanged between nodes Introduce a 2-level hierarchy with networks at the top level and hosts at the lower level – hierarchical aggregation Routers can have forwarding tables that list only a set of networks numbers rather than all nodes in each network IP: get IP datagram to the right physical network. Problem?
  9. 9. IP Addresses - Class A Start with binary 0 Range 1.x.x.x to 126.x.x.x Host portion is 24 bits All allocated
  10. 10. IP Addresses - Class B Start 10 Range 128.x.x.x to 191.x.x.x Second Octet also included in network address 214 = 16,384 class B addresses All allocated
  11. 11. IP Addresses - Class C Start 110 Range 192.x.x.x to 223.x.x.x Second and third octets also part of network address 221 = 2,097,152 addresses Nearly all allocated See IPv6
  12. 12. Internetworking Using IP (1) How do we get a datagram to a particular host or router on our network? The physical interface h/w on the host/router only understands the addressing scheme of that particular network (e.g., Ethernet flat addressing) IP address link-level address Encapsulate IP datagram inside a frame w Ethernet address Solution? Encode a host’s physical address in the host part of the IP address ⌧Class A/B/C? ⌧Ethernet? Network admin manually configures a table of mappings ARP ARP cache/table ⌧If (no mapping is found in ARP cache) Broadcast ARP query w target IP address
  13. 13. Internetworking Using IP (2) IP makes routing in an internet (or in the Internet) “SOMEWHAT” scalable Each router needs to know about all the networks connected to the internet Problem: today’s Internet has tens of thousands of networks Problem: large campus with many internal networks consider a Class C network with only 2/3 hosts consider a physical network that needs only 256/257 hosts
  14. 14. Internetworking Using IP (3) Conclusion: assigning one network number per physical network uses up the IP address space much faster than we would like. Plus, the more the IP net numbers, the larger the forwarding tables Solutions: Subnetting (for address utilization) Route propagation techniques (16.1)
  15. 15. Subnets and Subnet Masks (1) Take a single IP net number and assign it more than one physical network, or “SUBNET” They will all look as a single network Examples: large campus or corporation. Form the outside, all a router needs to know to reach any subnet is the IP net number Each physical net is assigned a subnet number The host portion of the IP address is partitioned into subnet number and host number Bit positions containing this EXTENDED net number are indicated by means of the “SUBNET MASK”
  16. 16. Subnets and Subnet Masks (2) What happens when a datagram arrives from the rest of the internet? The subnet MASK allows the host to send directly or send to a router Hosts need not employ a subnet MASK. (T/F) Hosts need not make routing decisions. (T/F) Different subnets, on the same internet, can have the same subnet MASK Forwarding table entries Are not of the form <subnet #, nextHop>. Why not? <subnet #, subnet MASK, nextHop>
  17. 17. Subnets and Subnet Masks (3) Different subnets, on the same internet, can have different subnet MASKs. Why? 255.255.255.192 255.255.255.224 Default subnet MASK for a given class of addresses is a null mask. What does this mean? [(Subnet #)*2k] + host # = host portion in IP portion 10*25 + 1 = 65 (host C on p.547) 1*25 + 25 = 57 (host B on p.547) Try it for hosts A and D For a Class C subnet MASK of 255.255.255.224, the first ___ rightmost/leftmost bits of the host portion of the IP address must be IDENTICAL for any 2 hosts on the same physical network
  18. 18. Subnets and Subnet Masks (4) For a Class C subnet MASK of 255.255.255.72, ___ bits of the host portion of the IP address must be IDENTICAL for any 2 hosts on the same physical network For a Class C subnet MASK of 255.255.255.0, the first ___ rightmost/leftmost bits of the host portion of the IP address must be IDENTICAL for any 2 hosts on the same physical network For a Class C subnet MASK of 255.255.255.224, how many different subnets can this accommodate? 2number of 1s in host portion of IP-like subnet mask
  19. 19. Routing Using Subnets

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