CST3607 Class Notes 2021-03-02

News & Tools

Subnetting Tutorial & Reference

Quiz #1 Debriefing

Quiz #2

  • 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,
    • and 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: Assignment #5 Subnetting Practice, Due Thurs. 2021-03-04

  • Subnet zero is allowed as per Cisco standard practice.
  • Use the bitwise AND function to determine network addresses.
  • Download the Assignment
  • Assignment #5, Due: Before Thurs. March 4, 6PM EST
  • No late assignments will be accepted


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