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- 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.
- 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 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.”
Read / Watch / Do
- Read Chapter 5 IP Routing
- Do the Written Labs
- Answer the Review Questions
- Do not submit your answers for this chapter. The answers are in Appendix.
- VLSM & Route Summarization (by The Networking Doctors)
- CCNA Training Part 46 Class less subnetting (by Lazaro Diaz / The Networking Doctors)