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Subnetting _ CIDR

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					Subnetting & CIDR

        Tahir Azim




    Courtesy Nick McKeown (Stanford), Umar Kalim
                       (NIIT)
             Announcements
• Participate in NASCON, FAST-NU Islamabad
• Assignment 1 deadline extended to Tuesday
  due to no BIT-7 classes on Monday
• From last time:
  – Packet bursting: An approach to increasing the speed
    of 802.11g-based wireless networks by unwrapping
    short 802.11g packets and rebundling them into a
    larger packet to reduce the impact of mandatory gaps
    between packets (jwire.com)


                  Courtesy Nick McKeown (Stanford), Umar Kalim
                                     (NIIT)
                     Subnetting
• Subnetting is a form of hierarchical routing.
• Subnets are usually represented via an address
  plus a subnet mask or “netmask”.
• e.g.
   nickm@elaine17.Stanford.EDU > ifconfig hme0
    hme0: flags=863<UP,BROADCAST,NOTRAILERS,RUNNING,MULTICAST>
  mtu 1500
    inet 171.64.15.82 netmask ffffff00 broadcast 171.64.15.255


• Netmask ffffff00: the first 24 bits are the subnet
  ID, and the last 8 bits are the host ID.
• Can also be represented by a “prefix + length”,
  e.g. 171.64.15.0/24, or just 171.64.15/24.
                      Courtesy Nick McKeown (Stanford), Umar Kalim
                                         (NIIT)
                                         Subnetting

                                     2                       14                            16
             CLASS “B”
                                    10                  Net ID                           Host-ID
            e.g. Company


              2         14                    16                                    2            14                      16
e.g. Site    10        Net ID      0000      Host-ID                               10           Net ID      1111         Host-ID

                  Subnet ID (20)                 Subnet
                                               Host ID (12)
                                                                                           Subnet ID (20)                   Subnet
                                                                                                                          Host ID (12)




              2         14                    16                                     2           14                      16

e.g. Dept    10        Net ID      000000          Host-ID                          10          Net ID      1111011011        Host-ID

                  Subnet ID (22)                 Subnet                                    Subnet ID (26)                       Subnet
                                                                                                                              Host ID (6)
                                               Host ID (10)




                                            Courtesy Nick McKeown (Stanford), Umar Kalim
                                                               (NIIT)
Routing in the presence of subnets
• The rest of the
  Internet is not aware
  of subnets within a
  network
• Levels: site, subnet,
  host
• Routing now involves
  delivery to the site,
  then the subnet and
  finally the host

                 Courtesy Nick McKeown (Stanford), Umar Kalim
                                    (NIIT)
                   Example of subnetting
To: cenic.net
                   hpr1-rtr      171.64.1.131                                              171.64.74.0/24
                                   171.64.1.132/30
                                  171.64.1.133

                                   171.64.1.161 171.64.1.178                          171.64.74.1
     Class B
     Address                bbr2-rtr                                        Gates-rtr
                                          171.64.1.160/27                                           171.64.74.58
  171.64.0.0/16
  AS 32                                                                                              EndHost
                                 171.64.1.152
                                       171.64.1.144/28
To: cogentco.com
                   border2-rtr   171.64.1.148




                                       Courtesy Nick McKeown (Stanford), Umar Kalim
                                                          (NIIT)
Classless Interdomain Routing (CIDR)
                                   Addressing
   The IP address space is broken into line segments, or blocks
        e.g. Block of 2 addresses, block of 128 addresses etc.
   Each block is described by a prefix.
   A prefix is of the form x/y where x indicates the prefix of all addresses
    in the block, and y indicates the length of the prefix.
   e.g. The prefix 128.9/16 represents the block containing addresses in
    the range: 128.9.0.0 … 128.9.255.255.

                          128.9.0.0                                    142.12/19
              65/8
                                        128.9/16

     0                                                                             232-1
                                            216

    128.9.16.14
                              Courtesy Nick McKeown (Stanford), Umar Kalim
                                                 (NIIT)
Classless Interdomain Routing (CIDR)
                         Addressing

                       128.9.19/24
                                  128.9.25/24

                   128.9.16/20 128.9.176/20

                                    128.9/16

     0                                                             232-1


 128.9.16.14

         Most specific route = “longest matching prefix”


                    Courtesy Nick McKeown (Stanford), Umar Kalim
                                       (NIIT)
Classless Interdomain Routing (CIDR)
                            Addressing

 Prefix aggregation:
    If a service provider serves two organizations with
     prefixes, it can (sometimes) aggregate them to form a
     shorter prefix. Other routers can refer to this shorter
     prefix, and so reduce the size of their address table.
    E.g. ISP serves 128.9.14.0/24 and 128.9.15.0/24, it
     can tell other routers to send it all packets belonging
     to the prefix 128.9.14.0/23.
 ISP Choice:
    In principle, an organization can keep its prefix if it
     changes service providers.

                       Courtesy Nick McKeown (Stanford), Umar Kalim
                                          (NIIT)
      Size of the Routing Table at
        the core of the Internet




Source: http://www.cidr-report.org/

                            Courtesy Nick McKeown (Stanford), Umar Kalim
                                               (NIIT)
                    Prefix Length Distribution
                    100000
Number of entries

                     80000
                     60000
                     40000
                     20000
                              0

                                      8     11 14 17 20 23
                                          Prefix length (bits)
         Source: Geoff Huston, Jan 2006



                                          Courtesy Nick McKeown (Stanford), Umar Kalim
                                                             (NIIT)
Examples




Courtesy Nick McKeown (Stanford), Umar Kalim
                   (NIIT)
       Finding the first address
• What is the first address in the block if one of the
  addresses is 167.199.170.82/27?

• Solution: The prefix length is 27, which means
  that we must keep the first 27 bits as is and
  change the remaining bits (5) to 0s. The
  following shows the process:

   Address in binary: 10100111 11000111 10101010 01010010
   Keep the left 27 bits: 10100111 11000111 10101010 01000000
   Result in CIDR notation: 167.199.170.64/27

                      Courtesy Nick McKeown (Stanford), Umar Kalim
                                         (NIIT)
       Finding the first address
• What is the first address in the block if one of the
  addresses is 140.120.84.24/20?




                  Courtesy Nick McKeown (Stanford), Umar Kalim
                                     (NIIT)
   Finding the last address in the
                block
• To the first address, add the number of
  addresses, minus one
• OR
• Set all bits that are not part of the CIDR
  prefix to 1




                Courtesy Nick McKeown (Stanford), Umar Kalim
                                   (NIIT)
                 Example
• Find the number of addresses in the block
  if one of the addresses is
  140.120.84.24/20.
• Solution: The prefix length is 20. The
  number of addresses in the block is 232−20
  or 212 or 4096. Note that this is a large
  block with 4096 addresses.


               Courtesy Nick McKeown (Stanford), Umar Kalim
                                  (NIIT)
                     Example 2
• Find the last address in the block if one of the
  addresses is 140.120.84.24/20.

• Solution
   – We found in the previous examples that the first
     address is 140.120.80.0/20 and the number of
     addresses is 4096. To find the last address, we need
     to add 4095 (4096 − 1) to the first address.
   – Or, set all bits that are not part of the CIDR prefix to 1
      • 140.120.(0101 1111)2. (1111 1111)2 = 140.120.95.255

                     Courtesy Nick McKeown (Stanford), Umar Kalim
                                        (NIIT)

				
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posted:9/23/2011
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