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Subnetting

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					Global Internet
             Scalability Issues
IP “hides” hosts in address hierarchy, but...
 Inefficient use of address space
    – class C network with 2 hosts (2/255 = 0.78% efficient)
    – class B network with 256 hosts (256/65535 = 0.39%
      efficient)
   Too many networks
    – today's Internet has tens of thousands of networks
    – routing tables do not scale
    – route propagation protocols do not scale
         Internet Structure
Recent Past   Stanford                             NSFNET Backbone
                                                                                                             ISU

                         BARNET                                                         MidNet
                         Regional                                                       Regional
                                                 Westnet
                                                 Regional
              Berkeley
                            PARC                                                  UNL                   KU
                                          NCAR                    UNM

                                                    UA




                                                                    Service Provided
                                                                       Backbone

                          Stanford
                                                                                                             ISU



Today                                BARNET
                                     Regional
                                                         Westnet
                                                         Regional
                                                                                             MidNet
                                                                                             Regional

                         Berkeley
                                        PARC                                           UNL
                                                 NCAR                   UNM                             KU

                                                             UA
               Subnetting
 Add another level to address/routing
  hierarchy: subnet
 Subnet masks define variable partition of
  host part of class A and B addresses
 Subnets visible only within site

            Network Number          Host Number

                       Class B address


            111111111111111111111111        00000000

                  Subnet Mask (255.255.255.0)


            Network Number      Subnet ID       Host ID

                      Subnetted Address
            Subnet Mask: 255.255.255.128

            Subnet number: 128.96.34.0
                                                                                       Subnet
128.96.34.15
                               128.96.34.1
                                                                                      Example
             H1
                          R1

                                         Subnet Mask: 255.255.255.128
            128.96.34.130
                                         Subnet number: 128.96.34.128



                                                      128.96.34.139
                  128.96.34.129
                                                    H2
                                 R2
H3
                                      128.96.33.1
     128.96.33.14


       Subnet Mask: 255.255.255.0

       Subnet number: 128.96.33.0



        Forwarding table at router R1
                    Subnet Number            Subnet Mask                Next Hop
                    128.96.34.0              255.255.255.128            interface 0
                    128.96.34.128            255.255.255.128            interface 1
                    128.96.33.0              255.255.255.0              R2
      Forwarding Algorithm
D = destination IP address
for each entry < SubnetNum, SubnetMask, NextHop>
   D1 = SubnetMask & D
   if D1 = SubnetNum
      if NextHop is an interface
         deliver datagram directly to destination
      else
         deliver datagram to NextHop (a router)
                      Notes
   Would use a default router if nothing matches
   Not necessary for all ones in subnet mask to be
    contiguous
   Can put multiple subnets on one physical network
   Subnets not visible from the rest of the Internet
   Perform longest prefix match when there are two
    entries in the forwarding table that match
    (128.35.42.1 matches 128.35.0.0-255.255.0.0 and
    128.35.42.0-255.255.255.0, but the second would
    be chosen)
               Numbers
 www.icann.org Internet Corporation for
  Assigned Names and Numbers
 www.arin.net is our authority and has more
  details
 Names and numbers have been privatized.
  The US government used to allocate them
The big picture
Current
Destinations
Host Counts



          QuickTime™ and a
TIFF (Uncompressed) decompressor
   are needed to see this picture.
     Top Level Domain Host Count
               Jan 2003

Domain        Hosts         L2 Domains     L3 Domains
net           61,945,611    181,746        12,354,344
com           40,555,072    1,716,051      12,594,151
jp            9,260,117     13,889         141,443
edu           7,459.219     3,994          2,533,055

http://www.isc.org/ds/WWW-200301/dist-bynum.html
Path Length
         Supernetting

 Assign block of contiguous network numbers to
  near-by networks
 Called CIDR: Classless Inter-Domain Routing
 Represent blocks with a single pair
      <first_network_address, count>
 Restrict block sizes to powers of 2
 Use a bit mask (CIDR mask) to identify block size
 All routers must understand CIDR addressing
       Route Aggregation with CIDR
                                              Corporation X

                                         (11000000000001000001)



   Border gateway
                      Regional network
(advertises path to

11000000000001)


                                              Corporation Y

                                         (11000000000001000000)
Two Autonomous Systems

        R1
                           R3




             R2
                  Autonom ous s ystem 1



             R4
                  Autonom ous s ystem 2



        R5                 R6
           Route Propagation

Idea: Impose a second hierarchy on the network that
  limits what routers talk to each other. (The first
  hierarchy is the address hierarchy that governs how
  packets are forwarded.)
 Autonomous System (AS)
    – corresponds to an administrative domain
    – examples: University, company, backbone network
    – assign each AS a 16-bit number
   Two-level route propagation hierarchy
    – interior gateway protocol (each AS selects its own)
    – exterior gateway protocol (Internet-wide standard)
    Popular Interior Gateway
           Protocols
   RIP: Route Information Protocol
    –   developed at Berkeley
    –   distributed with Unix
    –   distance-vector algorithm- neighbors
    –   based on hop-count
   OSPF: Open Shortest Path First
    –   recent Internet standard
    –   uses link-state algorithm-bcast
    –   supports load balancing
    –   supports authentication
    Multibackboned Internet
                    Large corporation

                                                   “Consumer ” ISP

Peering
point
                       Backbone service provider          Peering
                                                          point
  “ Consumer” ISP

                                          “Consumer”ISP
                Large corporation

   Small
corporation
          EGP: Exterior Gateway
                Protocol
   Overview
    – designed for tree-structured Internet
    – concerned with reachability, not optimal routes
   Protocol messages
    – neighbor acquisition: one router requests that another be its
      peer; peers exchange reachability information
    – neighbor reachability: one router periodically tests to see if the
      other router is still reachable; exchange HELLO/ACK
      messages; uses a k-out-of-n rule
    – routing updates: peers periodically exchange their routing tables
      (distance-vector)
                EGP Example
                  Exterior Neighbor (Other system)

N1                                     Source Net: N1
                                       G1
      G1                    G2
N2                          N3         1 N2
                 G3
                                       G2
           N4                          1 N3

                                       G3
           G4         G5
 N5                        N6          1 N4

                                       2 N5
                                       2 N6
       BGP-4: Border Gateway
              Protocol
Assumes the Internet is an arbitrarily interconnected set of
  AS's (Autonomous Systems). Define local traffic as traffic
  that originates at or terminates on nodes within an AS, and
  transit traffic as traffic that passes through an AS, we can
  classify AS's into three types:
 Stub AS: an AS that has only a single connection to one
  other AS; such an AS will only carry local traffic.
 Multihomed AS: an AS that has connections to more than
  one other AS, but refuses to carry transit traffic.
 Transit AS: an AS that has connections to more than one
  other AS, and is designed to carry both transit and local
  traffic.
     Autonomous System (AS)
 Each AS has:
 One or more border routers
 One BGP speaker that advertises:
    – local networks
    – other reachable networks (transit AS only)
    – gives path information
   Still pass information about every network
                     BGP Example
                                             Customer P   128.96
                                              (AS 4)      192.4.153
                       Regional Provider A

                             AS 2
                                             Customer Q   192.4.32
                                               (AS 5)     192.4.3
"Backbone" Network
      AS 1

                       Regional Provider B   Customer R   192.12.69
                                               (AS 6)
                             AS 3


                                             Customer S   192.4.54
                                               (AS 7)     192.4.23
                BGP Example
   Speaker for AS 2 advertises reachability to P and
    Q
    Network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be
     reached directly from AS 2.
   Speaker for backbone network then advertises
    Networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can
     be reached along the path <AS 1, AS 2>.
   Speaker can also cancel previously advertised
    paths
Domain Divided into areas
                                             Area 3
      Area 1
                          Area 0
 R9            R7
       R8           R1                  R3
                                              R4
                              R2
                               Area 2


                         R6        R5
              Routing Basics
   Minimize the size of routing tables
    – Create Autonomous routing systems
   Simplify routing
    – hierarchical routing
    – Optimize within the Autonomous system

				
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posted:9/26/2011
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