Subnet
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Subnet

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Subnet Subnet Presentation Transcript

  • Data Networking Year 2
      • Subnetting
      • Colm Bennett
  • From Class to Subnet
    • Basic Class A, B, C was very wasteful of IP addresses
    • Also difficult to manage internally
    • In theory a company that needs 5000 IP adresses should get a Class B
      • But this wastes ~60,000 ip addresses
      • Plus they probably don't manage those 5000 as one network
    • So they tended to ask for multple Class C's
      • Harder to control internally
      • Starts to clog up routing tables on the internet
  • Partial Solution - Subnetting
    • A subnet is a sub network within one of the normal “classful” networks
    • Done by specifying a subnet ID within the host part of the IP address
    • So it robs some of the bits from the host part to use as a subnet ID
    • Specified as a custom Subnet Mask
    • Allows large Class A or B networks to be managed more efficiently internally
  • Subnetting
    • Obviously required protocols to understand this convention
      • RIP 2 for example
  • Designing a Subnet Solution
    • Establish the number of physical networks required
    • Subnet the network to give at least this number of networks
    • Confirm that the number of hosts left per subnet is OK
  • Design cont.
    • Subnet Ids of all 0's and all 1's are not normally used
      • Similar reasoning to Host Ids
    • So possibly number of subnets is 2^n – 2 where n = number of bits taken from the host field
    • So a 4 bit subnet mask will allow for (2*2*2*2)-2 = 16-2 = 14 subnets
  • Class B Possible Subnets – You Do!
  • Example
    • Class B 153.74.0.0
    • Company has 50 physical networks, max 200 hosts on any one
    • Looking at chart, taking 6 bits would give 62 possible subnets
      • 1022 Hosts per subnet so OK
      • Robbing 6 bits means 3 rd octet of Subnet Mask becomes
        • 1111 1100
        • = 128 + 64 + 32 + 16 + 8 +4 = 252
      • Subnet Mask = 255.255.252.0
  • Practical
    • Irish company has office in each county on the island
    • Need 6 hosts in each office
    • Will a Class C network do?
  • Partial Solution?
    • Only a partial solution
      • Solved the internal management issue
      • Stopped medium size companies requesting many Class C's
      • But still left those companies being assigned a Class B and wasting 1000's of IPs
      • Needed some way of moving beyond the whole “Classful” approach - CIDR/VLSM