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Internetworking - Internet Address Depletion and CIDR Introduction

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					                                                                 Introduction
                                                ►A  subnet is a subset of class A, B, or C
                                                  networks
   Internet Address Depletion
                                                ► IP addresses are formed of a network and
            and CIDR                              host portions – network mask used to
                                                  separate the information




              Introduction                                       Introduction
► Each class of address has its own “natural    ► Mask of 255.255.0.0 is applied to network
 mask” – mask created by the definition of       10.0.0.0
 the network
   class A natural mask 255.0.0.0                    divides the IP address 10.0.0.1 into a network
   class B natural mask 255.255.0.0                  portion of 10, subnet portion of 0, host portion
   class C natural mask 255.255.255.0                of 0.1
► By using masks, networks can be divided
 into subnetworks
   extends the network portion of the address
   into host portion
   increases the number of subnetworks and
   reduces the number of hosts




                                                                                                        1
 Variable Length Subnet Mask                                 Variable Length Subnet Mask
► VLSM  allows a network to be be
 configured with different masks
   adds more flexibility in dividing the network
   into multiple subnets
   without VLSM a mask may have too few
   subnets or hosts
► Suppose  we want to split 192.214.11.0
 (class C) into three subnets with 100 hosts
 in one subnet and 50 hosts in each
 remaining subnet




                      CIDR                                                      CIDR
          Inter-Domain Routing was
► Classless                                                 ► Classless   addresses
 designed as a remedy for                                      main observation: many organizations need
   class B exhaustion                                          more than a class C network but does not
                                                               have enough hosts to efficiently utilize a class
   routing table explosion                                     B
    • as more networks get connected -- more memory            idea: give such organizations multiple class C
      is needed for storing routing tables                     addresses
    • most high performance routers “cache” portions of        in the CIDR strategy, the class C addresses
      routing tables at the interface board themselves --      are contiguous and share the same “most
      to speedup forwarding                                    significant bits” -- the same prefixes
    • some extreme designs had fast memories that              if the routing protocols can route based on
      were in stand-alone mode at the interface boards         these prefixes, they need only one block of
                                                               network numbers




                                                                                                                  2
                        CIDR                                                CIDR
       by allocating addresses intelligently -- we can   ► CIDR  notation enables lumping of specific
       group numbers by region                             routes into aggregates
► InCIDR, an IP network is represented by                ► Aggregate denotes any summary route
  a prefix
                                                         ► Supernet denotes a summary route with
       IP address + some indication of the left-most
                                                           shorter prefix length than the natural mask
       contiguous significant bits within this address
►A  network is called “supernet” when prefix
  boundary contains fewer bits than the
  networks natural mask




                        CIDR                                                CIDR
                                                         ► Networks    that are subset of an aggregate
                                                           or a CIDR block are called “more specific”
                                                         ► Routing domains that are CIDR-capable
                                                           are called “classless” – traditional routing
                                                           “classfull” routing




                                                                                                          3
     Route Aggregation in CIDR                             Route Aggregation in CIDR
                                                      ► Aggregation    may not work always
                                                           customers having IP addresses that do not
                                                           belong to their provider’s range
                                                           some customers (ISPs) need to connect to
                                                           multiple providers at the same time
                                                      ►A  router with 198.32.1.0/24 and
                                                       198.32.0.0/16 will match 198.32.1.0 when
                                                       trying to deliver traffic to 198.32.1.1




                                                       Single Homing: Address Outside
          Longest Prefix Match
                                                          Provider’s Address Space
►   Destinations connected to multiple domains        ► Customer  connected to single provider
    must be explicitly announced – in most specific   ► IP address space different from provider’s
    forms
                                                      ► Customer changed providers and kept
                                                        addresses of the previous provider
                                                      ► Renumbering should be done – if not
                                                           provider cannot aggregate as efficiently – hole
                                                           is punched in the address space
                                                           new provider cannot aggregate the address
                                                           either




                                                                                                             4
    Multihoming Scenerio: Addresses                         Multihoming Scenerio: Addresses
         taken from one provider                                 taken from one provider
► Customers   are connected to multiple
  providers – small enough to take
  addresses only from one
► Aggregate advertisement can lead to black
  holes
► Aggregating someone else’s routes (proxy
  aggregation) can be tricky
      unless aggregating party is a superset
      or parties are in total agreement




    Multihoming Scenerio: Addresses                         Multihoming Scenerio: Addresses
         taken from one provider                                 taken from one provider
►   ISP2 sends an aggregate summarizes Jamesnet
    and Lindanet into one update 198.24.0.0/18
►   Stubnet which is a customer for ISP1 has an
    address space falling in 198.24.0.0/18
►   Traffic for Stubnet 198.24.16.0/21 will perform
    longest match and endup in ISP2
►   Solution:
      ISP2 should specifically list each of the IP ranges
      that it has in common with ISP1 on top of its own
      address space 198.32.0.0/13




                                                                                              5
Multihoming Scenerio: Addresses       Multihoming Scenerio: Addresses
     taken from one provider           taken from Different Providers
                                  ►   Large domains can take addresses from
                                      different providers
                                  ►   Each provider aggregates its own address
                                      space without listing specific ranges from other
                                      provider
                                        drawback – backup routes to multihomed
                                        organizations not maintained – redundancy is one of
                                        the reasons for multi-homing!
                                        traffic using the addresses taken from provider will be
                                        unable to reach the destination if the provider is down
                                        – even if the destination is reachable via “other”
                                        provider




Multihoming Scenerio: Addresses
 taken from Different Providers




                                                                                                  6

				
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