CST3607 Class Notes 2018-10-02

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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 each subnet,
    • and all the subnets have the same number of addresses in them.

Classless addressing allows you to use different subnet masks and create 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 192.168.10.0/24, 192.168.11.0/24 and 192.168.12.0/24 could all be summarized into 192.168.8.0/21.

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
    • RIPv2, EIGRP, OSPF, BGP
  • 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 decending 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.”

Watch:

Do

  • Assignment #4: VLSM Problems from the handout received in class on this day.
    • Due Thurs. 10/4/2018
    • 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

Quiz #1: Subnetting

  • (10 questions, 1/2 hour only)
  • Make sure to bring in your stand-alone calculator, pencils, and an eraser.
  • Make sure that you’re proficient and comfortable with the decimal to Base-256 dotted-decimal conversion method. Practice makes improvement!
    • “Subnet number” x starts from 0, so do not subtract one before you multiply by the number of addresses per subnet. The result will be the number of addresses to add to the first address in the block to get to the first address in subnet #.
      • For example: What’s the subnet range for subnet “number” 1023?
      • i.e. The 1024th subnet is subnet number 1023.
    • The “Nth subnet” starts from 1, so you’ll have to subtract one before you multiply by the number of addresses per subnet. The result will be the number of addresses to add to the first address in the block to get to the first address in the Nth subnet.
      • For example: What’s the subnet range for the 5028th subnet?

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