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Chapter Four

Chapter Four. Subnetting. Objectives. Understand why subnetting is necessary Describe how computers work with binary TCP/IP addresses Subnet any network Determine the number of useable hosts on a subnet Supernet several smaller networks. Reasons to Subnet. Subnetting

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Chapter Four

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  1. Chapter Four Subnetting

  2. Objectives • Understand why subnetting is necessary • Describe how computers work with binary TCP/IP addresses • Subnet any network • Determine the number of useable hosts on a subnet • Supernet several smaller networks

  3. Reasons to Subnet • Subnetting • The process of separating a network into several smaller networks to improve performance • Subnetting is used because it can • Reduce collisions • Limit broadcasts • Control traffic

  4. Reducing Collisions • Collision • When two computers on an Ethernet network using CSMA/CD as an access method attempt to transmit at the same time • Subnetting • Reduces number of hosts on each network • Reduces the amount of traffic on the network • With less traffic, collision is reduced • Collisions that occur on one network do not affect another network

  5. Limiting Broadcasts • Subnetting a network • Creates multiple networks with fewer hosts on each network • Fewer hosts results in fewer broadcast messages • Routers • Packets addressed to IP address 255.255.255.255 are not forwarded

  6. Controlling Traffic • Routers • Allows a greater degree of control over network traffic • Most have ability to implement rules about which packets they forward

  7. Binary and TCP/IP • IP addresses • Internally, computer looks at an IP address as a single group of 32 binary digits • Decimal numbering • A base-ten numbering system • Uses the values from 0 to 9 for each digit

  8. Counting in Decimal

  9. Decimal Column Values

  10. Binary Numbering • Subnetting • Based on binary • Binary • Base two numbering system • Two potential values for each digit: 0 and 1

  11. Binary Counting

  12. Binary Column Values

  13. Conversion Between Binary and Decimal • To convert a binary octet to a decimal value • Multiply the digit in each column by the value of each column • Then determine the sum of those products

  14. Binary to Decimal Conversion

  15. Windows Calculator in Scientific Mode

  16. Binary Subnet Mask • Computer calculates the host ID and network ID of an IP address while working in binary • Part of the network ID • Where there is a 1 in the subnet mask • Part of the host ID • Where there is a 0 in the subnet mask • ANDing • Binary process used by computers to find the network ID • An IP address ANDed with a subnet mask gives the network ID • 1s in a subnet mask must be contiguous

  17. Calculating Host ID and Network ID

  18. Invalid Subnet Masks

  19. Subnetting a Network • To subnet a network • Take some bits from the host ID and give them to the network ID • Class B addresses • Normally very large • Generally need to be subnetted to handle routing between different physical locations • Using an entire octet for subnetting gives 256 possible subnets • Traditionally, subnets with all 1s and all 0s are discarded, leaving 254 usable subnets

  20. Simple Subnetting

  21. Complex Subnetting

  22. Subnet Hosts • 2n-2, where n is the number of host bits • Formula used to calculate the number of subnets that can be created from a certain number of bits • Also used to calculate the number of usable hosts on a subnet

  23. Usable Hosts

  24. Usable Hosts Formula

  25. Supernetting • The opposite of subnetting • Used to create one large network from several smaller ones • Takes bits from network ID and gives them to the host ID • All networks being combined for supernetting must be contiguous • IP addresses from first network to last must be one single range with no breaks

  26. Supernetting Two Class C Networks

  27. Supernetting Four Class C Networks

  28. Reasons for Supernetting • Supernetting • Used when a range of IP addresses larger than a Class C network is required, but a full Class B network is not required • May also be used to reduce routing complexity

  29. Summary • Subnetting • Used to divide a single large network into multiple smaller networks • Reduces packet collisions • Limits broadcasts • Controls network traffic • Binary • Base-two numbering system • Only 0 and 1 are valid values • Computers • Work with IP addresses as 32-digit binary numbers

  30. Summary (Continued) • Subnet Mask • 1 corresponds with bit that is part of network ID • 0 corresponds with bit that is part of host ID • Subnetting • Takes bits from host ID and uses them as part of network ID • Number of useable hosts on a network • Calculated with the formula 2n-2 • n is the number of host bits • Supernetting • Combines multiple smaller networks into a single larger network • Networks being combined must be contiguous

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