CST3607 Class Notes 2019-02-28

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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 in them.

Classless addressing allows you to use different a subnet mask for each subnet, thus creating subnets tailored to the number of users/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 Numeric Method Outline

  • 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.”



Assignment #5: VLSM Problems from the handout received in class today

  • Due Tues. 2019-03-05
  • Make sure to write as neat as possible so that I can read your answers.
    • Do your calculations/work/notes on a copy or separate paper, then neatly copy your answers to the assignment handout.
  • Enter you answers in pencil, in case you have to make changes.
    • If you insist on using a pen, and you have to make changes, use whiteout instead of crossing out and making a mess.
  • 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.
  • 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.
    • The total addresses per subnet must be a power of two.
    • Each subnet must be a power of two.
  • Determine the broadcast address for a subnet
    • Add 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: A to B, B to C, C to D. i.e. A–B–C–D
    • Draw your diagram on a separate sheet of paper, that you should attach to the assignment you hand in.
  • Make a copy of your completed assignment, so that you can follow along during class.
  • No late assignments will be accepted

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