Subnet

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Subnet

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

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