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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.
- This technique is referred to as VLSM, Variable Length Subnet Masks.
- 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.
- (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
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.”
- VLSM & Route Summarization (by The Networking Doctors)
- CCNA Training Part 46 Class less subnetting (by Lazaro Diaz / The Networking Doctors)
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