CST3607 Class Notes 2021-09-30

News & Tools

Subnetting Tutorial & Reference

  • Make sure to read the Instructions containing the information need to answer the questions. If you’re confused/unsure about something, re-read the instructions.

Classful vs. Classless Subnetting

When you’re subnetting an IP address for a network you have two options: classful and classless.

Classful subnetting is the simplest method.

  • It tends to be the most wasteful because it uses more addresses than are necessary.
  • In classful subnetting you use the same subnet mask for every subnet, so all the subnets have the same number of addresses per subnet.

Classless Subnetting allows you to use different a subnet mask for each subnet, thus creating subnets tailored to the number of addresses in each group.

VLSM (Variable Length Subnet Mask)

  • VLSM (Variable Length Subnet Mask) is a way of further subnetting a subnet.
  • In previous lessons, we divided a network only into subnets with an equal number of IPv4 addresses.
  • Using Variable Length Subnet Masking (VLSM) we can allocate IPv4 addresses to the subnets by the exact need.
  • Variable Length Subnet Masking (VLSM) allows us to use more than one subnet mask within the same network address space.
  • Variable Length Subnet Masking (VLSM) allows us to create subnets from a single network with an unequal number of IPv4 addresses.
  • VLSM supports hierarchical addressing design therefore, it can effectively support route aggregation, also called route summarization.
  • Route summarization can successfully reduce the number of routes in a routing table by representing a range of network subnets in a single summary address. For example subnets, and could all be summarized into

VLSM: Configuring Subnets Using the Numeric Method (PDF)

  • (Insert Chart: # of Hosts, # of Subnets, Binary Values, Mask)
  • (Insert Chart: Prefix, Mask, Subnets, Hosts, Block Size)
  • Routing Protocols that do not support VLSM
    • RIP, IGRP
  • Routing Protocols that do support VLSM
  • Supernetwork

VLSM using the Numeric Method

  • Draw a chart with the
    • “Number of Hosts” (line 1),
    • “Number of Subnets” (line 2),
    • “Binary Values” (line 3),
    • “Bit Values” (line 4)
  • Determine the number of addresses needed for all subnets, and write them in descending order.
  • Draw a line allowing for the number of hosts/addresses needed, and label it.
  • Use the value of the last bit borrowed (line 2, # of subnets) or the Block size as increment to the next subnet.
  • The broadcast address is one less than the “next subnet.”

Do Now



Do: Assignment #5 VLSM, Due Tues. 2021-10-05

  • Subnet zero is allowed as per Cisco standard practice.
  • Use the bitwise AND function to determine network addresses.
  • Download the Assignment
  • Due: Before Tues. 10/5, 6PM EST
  • No late assignments will be accepted
  • VLSM: Configuring Subnets Using the Numeric Method (PDF)
  • VLSM Addressing Samples (PDF)
  • IPv4 VLSM Addressing Worksheet (rtf)
  • You must start from the largest subnet to the smallest subnet.
  • The “# hosts” column must be the “number of needed hosts” specified in the problem, not the power of 2.
  • You must base your calculations on the total addresses per subnet.The problems ask for the number of needed hosts, i.e. usable addresses, so don’t forget to include the subnet and broadcast addresses when you specify the subnet range.
  • Determine the broadcast address for a subnetAdd the Wildcard mask to the subnet address to get the broadcast of the subnet.
  • Or, you can add the block size to increment to the next subnet, then subtract one to get the broadcast address of the previous subnet.
  • Problem 31:Requires 1 router for each location, and each router connects to 1 switch for the location.
  • The routers use a point-to-point connection: R1 to R2 to R3 to R4. i.e. R1–R2–R3–R4
  • Draw your diagram on a separate sheet of paper, that you should attach to the assignment you hand in.
  • Problem 33:“One router with four Ethernet ports will be used for this network.”
  • Each interface on the router would connect to each of the four subnets, using one IP address from each subnet.
  • Make a copy of your completed assignment, so that you can follow along during class.
  • No late assignments will be accepted
  • Download the Assignment


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