Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 471Recall Chapter 3:Figure 3.2We need an addressing system that separately identifies the destination network and the destination hostso that routers can forward on basis of destination network.Each network needs a distinct network address.2Recall Chapter 4: every physical network has to have its own unique IPv4 network address (class A, B, or C)Figure 4.1The classful system is rigid and wasteful of the IP address space.Efforts to utilize the IPv4 address space more efficiently and delay exhaustion:► 1987 subnetting (chapter 9A)► 1993 supernetting (chapter 9B)► 2000 network address translation (chapter 19)With the advent of Local Area Networks and growth of the Internet, it became clear that the IPv4 address space would “soon” become exhausted.3Chapter 9 – Classless and Subnet Address Extensions (CIDR)9.3 Minimizing Network NumbersWe have been assuming that every physical network had its own class A, B, or C IP network prefix.UAB has class B network address 138.26.0.0But there is clearly more than one physical network on campus!9.5 Subnet AddressingThis became a required part of IP in 1987.Individual sites have the freedom to modify addresses and routes as long as the modifications remain invisible to other sites.A site can choose to use IP addresses in unusual ways internally as long as:► all hosts and routers at the site agree to honor the site’s addressing scheme;► other sites on the Internet can treat addresses as a network prefix and a host suffix.49.5 Subnet Addressing - continuedExample:Site with class B address 128.10.0.0 has two physical networks - subnetsOnly router R knows that there are multiple physical networks at the site and how to forward traffic among them; all other routers in the Internet forward traffic as if there were a single physical network at the site.Figure 9.259.5 Subnet Addressing - continuedIn this example the local site has chosen to use the third octet of the address to distinguish between the two networks.“Subnetting on the basis of the 3rd octet”69.5 Subnet Addressing – continuedRelationship to original “classful” addressing systemFigure 9.3Original classful IP addressingWith subnet addressingHierarchical routing:Routers in other autonomous systems use only the Internet part;Routers within this autonomous system also use the additional octet;Final delivery to host also uses the remaining octet.subnet79.6 Flexibility in Subnet AssignmentSubnetting on the basis of the third octet is not the only possibility. Consider this intranet:Figure 9.4Need 5 subnets - How could subnet addressing be performed?89.6 Flexibility in Subnet Assignment – continuedThe number of bits allocated to the subnet can be chosen to fit the situation. Slice up the pie for a class-B address:9In this example, allocating 3 bits would allow 6 subnets (23 – 2), each with up to 8190 (213 – 2) hosts.( -2 because we do not assign all-zeros or all-1s to hosts or subnets)Figure 9.4109.8 Implementation of Subnets with MasksThe division of the 32-bit IP address for subnetting is controlled by the 32-bit subnet mask or address maskNetwork + subnet part Host partWe use dotted decimal notation to represent this as 255 . 255 . 255 . 0To extract the (network + subnet) part of an IP destination address (as needed for routing within intranet): <destination IP address> .AND. <subnet mask>For example, subnetting on the basis of the third octet is represented by11111111 11111111 11111111 0000000011Recall from chapter 7 – Forwarding IP Datagrams:Figure 7.2129.11 The Subnet Forwarding AlgorithmBefore subnetting, the rows of the routing table contained duples <destination network address> <next hop>We extracted the destination network address from the 32-bit datagram destination address by looking at the first 1, 2, or 3 bits, to identify if the destination host was connected to a class A, B, or C network.With subnetting it is no longer possible for internal routers to deduce the destination network address on the basis of the datagram destination address alone.We have to expand our routing table rows to triples <address mask> <destination network address> <next hop>and revise the forwarding algorithm used in the intranet.139.12 A Unified Forwarding AlgorithmRecall the Chapter 7 forwarding algorithm:Figure 7.3This algorithm has to be modified to account for subnetting.Before subnetting we could determine N from destination IP address alone149.12 A Unified Forwarding Algorithm – continuedFigure 9.7Only the internal routers have to be modified, since subnetting is invisible to outsiders.New stepThis also needs to be ANDed with the address mask159.12 A Unified Forwarding AlgorithmRecall the Chapter 7 forwarding algorithm:Figure 7.3What happened to these special cases?169.12 A Unified Forwarding Algorithm – continuedWhat happened to the special cases in figure 7.3?Default route: use address mask 0 . 0 . 0 . 0with destination network address 0 . 0 . 0 . 0 table row will be <address mask> <destination network address> <next hop> 0 . 0 . 0 . 0 0 . 0 . 0 . 0 next hop for default route (because any ID ANDed with 0 . 0 . 0 . 0 gives 0 . 0 . 0 . 0)Default route must be last row in forwarding table.Host-specific route: use address mask 255.255.255.25517Figure 9.4.254.253 .252.254.253.252Class B address 138.26.0.0All subnet masks 255.255.255.0138.26.1.0138.26.2.0.1138.26.3.0189.13 Maintenance of Subnet MasksSubnet masks are assigned by the network administrator.199.14 Broadcasting to SubnetsRecall special forms of IP address from chapter 4:Following this, we interpret an IP address<network part> <subnet part> 111…11as indicating broadcast to all hosts on <subnet>Figure 4.4209.15 Anonymous Point-to-Point NetworksA typical situation for a router in a wide-area network is that once an exit interface has been chosen, there is only one possible destination.In this situation we don’t need to waste an IP network prefixand don’t need to ARP.219.15 Anonymous Point-to-Point Networks - continuedFigure