CST3607 Class Notes 2021-09-11

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Subnetting Tutorial & Reference Page

Assignment #2 Debriefing

Subnetting into a Large Number of Subnets

  • Incrementing subnets using the Block Size works for a small number of subnets, but is not efficient when you need hundreds or thousands or millions of subnets. It doesn’t scale.
  • Using the Base-256 conversion scales.

Determine the network address of a high subnet number.

  1. Multiply the target subnet number by the number of addresses per subnet, to get the number of addresses to add to the network address (subnet zero) to jump to the target subnet.
  2. Convert the resulting number of addresses to its Base-256 (dotted-decimal) equivalent.
  3. Add the Base-256 (dotted-decimal) equivalent to the network address/subnet zero, to determine the target subnet address.

Notes about the “target subnet”

  • If you’re given subnet number x, then you use x as is to multiply by the number of addresses per subnet.
  • If you’re given the nth, subnet, e.g. 59th, 343rd, then you subtract one, then multiply by the number of addresses per subnet. (Because we start counting from zero.)

Converting a Decimal Number to Base 256  (Dotted-decimal)

Calculations for Base-256 Conversion
Evaluate the #Is the # greater than 256?
4th Octet.
3rd Octet.
2nd Octet.
1st Octet.

Subnetting Tips/Notes

  • If no mask/prefix is given, then borrow bits starting from the “Class” boundary of the IP address.
  • If a mask/prefix is given, then the given mask/prefix is the result of subnetting.  (Borrow bits from the “Class” boundary to the given mask/prefix.) (e.g. Q. 7, Pg. 40)
  • The total number of subnets and total number of hosts must be a power of 2.
  • Is the question asking for “subnets” or “hosts”
    • If you’re asked for the # of hosts, then you must determine how many bits are needed to get that # of hosts, then subtract those bits from the 32 IPv4 bits, to determine the network bits / mask / prefix.
  • Determine the number of subnets: 2[number of bits borrowed].
  • Determine the total number of addresses: 2[the number of host bits].
  • Add the Wildcard mask to the network/subnet address to determine the broadcast/last address in the network/subnet.
  • Block Size:
    • The block size (256 – [The interesting octet]) is best used to determine the increment of the subnets.
    • The interesting octet is the last octet, from the left, that you borrowed bits from.
    • The “block size” is not the number of addresses per subnet. It is the increment from one subnet to the next, within the “interesting” octet.
  • Determine how many addresses to add to the network address/subnet zero to get to the target subnet.
    • 1. Multiplying (Subnet “Number”) by the (number of addresses per subnet).
      (For the Nth subnet, subtract 1 before multiplying by the number of addresses per subnet.)
    • 2. Convert the result to its Base-256 equivalent
    • 3. Add the Base-256 equivalent to the original network address of the block to get the network/subnet address of the target subnet.
  • The “subnet address” is an alternate term for the “network address” of a subnet.
  • Subnet using the methods that work for all subnets, large or small. Switching methods depending on the size of the subnet requires more effort than is necessary.
  • Practice makes improvement!


Do: Assignment #2: Due before Tues. Feb. 23, 2020 6pm EST

  • Download the Assignment
  • Important: Make sure to read and understand the instructions on how to handle the protected PDF
  • If you have any issues completing all parts of every question on the assignment, e-mail me with the question # and the specifics you need assistance with.
  • No late assignments will be accepted.


Read / Watch / Do

CCNA Certification Study Guide, Volume 2

  • Read Chapter : 6 OSPF (Open Shortest Path First)
  • Do the Written Labs
  • Answer the Review Questions
    • Do not submit your answers for this chapter. The answers are in Appendix.


Make sure to always have access to a calculator which has an Exponent function (^key) ( x) for every class.

Better Focus and Efficient Studying When Not Multitasking