CST3607 Class Notes 2019-02-19

Assignment #1 Debriefing

Google: My Activity

  • See the logs of what you’ve searched for, visited, and watched on Google services.


  • Combines contiguous networks to create a larger block of addresses
  • Decreases the number of 1’s in the mask
    • (i.e. Decreases the number of “network” bits, and increases the number “host” bits)


  • Divides an address block into smaller networks
  • Increases the number of 1’s in the mask
    • (i.e. Increases the number of “network” bits, and decreases the number “host” bits)

Answer these questions when subnetting:

  • How many subnets?
  • How many addresses/hosts per subnet?
  • What are the valid subnets?
  • What’s the broadcast address for each subnet?

Subneting into a Large Number of Subnets

  • 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.

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.

Note 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.
  • Be conscious of whether you’re asked for “subnets” or “hosts.”
    • If you’re asked for the # of hosts, then you must determine how many bits are needed to get that # 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, withing 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” 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


Assignment #2, Due Tues. 2019-02-26

  • Subnetting problems 6 through 15 on the handout received in class today.
    • Notes:
      • If the prefix/mask is given, and either the required number of subnets, or the required number of host addresses, then the prefix/mask is our starting point. (Start borrowing bits after the given prefix)
      • If only the Network Address and a Prefix are given, then the given prefix is the result of an already subnetted network. Since we’re not given any other information, we’ll have to use the “class” of the Network Address as the starting point, before subnetting, and then borrow the number of bits needed to match the given prefix.
      • The nth subnet range, includes the subnet network IP address and the broadcast IP address. e.g. to
      • The subnet number for the nth subnet, is the 1st address in the nth subnet. e.g. Network ID / Subnet address / Network address for the nth subnet.
    • Do your calculations/work/notes on a copy or separate paper, then neatly copy your answers to the assignment handout.
    • The answers you hand in for your assignment must be neat and readable, by me. Use a pencil.
    • Use a pencil. Write neatly
      • Use a pencil. Write neatly.
        • Use a pencil. Write neatly.
    • If you have any issues completing all parts of every question on the assignment, e-mail me with the question # and specific issue you’re having difficulty with.
    • This assignment takes a lot of time to complete, so start on it right away. Give yourself enough time to complete all parts of every question, without rushing.
    • No late assignments will be accepted.
    • Note: We will go over this assignment in our next class, after you’ve handed in the hardcopy. 
      Make sure you make a copy of your completed assignment, so that you can follow along while we go over the assignment in class.
    • Review Subneting Examples 1 & 2
  • Until further notice, bring in a stand-alone calculator (not the one on your phone, or computer) to class. One that has an Exponents (^key) ( x)

Example: Decimal to Base-256/Dotted-Decimal Conversion (Subnetting Problem 2)


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