Chapter 8:
Subnetting IP Networks
CCNA Routing and Switching
Introduction to Networks v6.0
Chapter 8 - Sections & Objectives
8.1 Subnetting an IPv4 Network
• Implement an IPv4 addressing scheme to enable end-to-end connectivity in a small to
medium-sized business network.
• Explain how subnetting segments a network to enable better communication.
• Explain how to calculate IPv4 subnets for a /24 prefix.
• Explain how to calculate IPv4 subnets for a /16 and /8 prefix.
• Given a set of requirements for subnetting, implement an IPv4 addressing scheme.
• Explain how to create a flexible addressing scheme using variable length subnet masking (VLSM).
8.2 Addressing Schemes
• Given a set of requirements, implement a VLSM addressing scheme to provide
connectivity to end users in a small to medium-sized network.
• Implement a VLSM addressing scheme.
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Chapter 8 - Sections & Objectives (Cont.)
8.3 Address Schemes
• Explain design considerations for implementing IPv6 in a business network.
• Explain how to implement IPv6 address assignments in a business network.
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8.1 Subnetting an IPv4
Network
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Network Segmentation
Broadcast Domains
Devices use broadcasts in an Ethernet LAN to locate:
• Other devices - Address Resolution Protocol (ARP) which sends Layer 2 broadcasts to a known IPv4
address on the local network to discover the associated MAC address.
• Services – Dynamic Host Configuration Protocol (DHCP) which sends broadcasts on the local network
to locate a DHCP server.
Switches propagate broadcasts out all interfaces except the interface on which it was received.
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Network Segmentation
Problems with Large Broadcast Domains
Hosts can generate excessive broadcasts and negatively affect the network.
• Slow network operations due to the significant amount of traffic it can cause.
• Slow device operations because a device must accept and process each broadcast packet.
Solution: Reduce the size of the network to create smaller broadcast domains. These smaller
network spaces are called subnets.
One
Broadcast Broadcast in Broadcast in
Domain LAN 1 LAN 2
contained in contained in
1 subnet 1 subnet
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Network Segmentation
Reasons for Subnetting
Reduces overall network traffic and improves network performance.
Enables an administrator to implement security policies such as which subnets are allowed or not
allowed to communicate together. Communicating between
Networks
Subnetting by Location
Subnetting by Device Type
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Subnetting an IPv4 Network
Octet Boundaries
Networks
are most
easily
subnetted
at the
octet
boundary
of /8, /16,
and /24
Prefix length and the subnet mask are different ways of identifying the network
portion of an address.
Subnets are created by borrowing host bits for network bits.
More host bits borrowed, the more subnets that can be defined.
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Subnetting an IPv4 Network
Subnetting on the Octet Boundary
Subnetting Network 10.x.0.0/16
Define up to 256 subnets with each subnet capable of connecting 65,534 hosts.
First two octets identify the network portion while the last two octets are for host IP addresses.
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Subnetting an IPv4 Network
Subnetting on the Octet Boundary (Cont.)
Subnetting Network 10.x.x.0/24
Define 65,536 subnets each capable of connecting 254 hosts.
/24 boundary is very popular in subnetting because of number of hosts.
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Subnetting an IPv4 Network
Classless Subnetting
Subnetting a /24 Network
Subnets can borrow bits from any host bit position to create other masks.
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Subnetting an IPv4 Network
Video Demonstration – The Subnet Mask
Subnetting in Binary
ANDING
• Convert IP address and Subnet Mask to Binary (line up vertically like an addition problem)
• Logically AND (1 and 1 = 1, all other combinations = 0)
• Result is network address for original IP address
Classful Subnetting
• Class A /8 255.0.0.0
• Class B /16 255.255.0.0
• Class C /24 255.255.255.0
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Subnetting an IPv4 Network
Video Demonstration – The Subnet Mask (Cont.)
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Subnetting an IPv4 Network
Video Demonstration – Subnetting with the Magic Number
Magic number technique used to calculate subnets
Magic number is simply the place value of the last one in the subnet mask
/25 11111111.11111111.11111111.10000000 magic number = 128
/26 11111111.11111111.11111111.11000000 magic number = 64
/27 11111111.11111111.11111111.11100000 magic number = 32
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Subnetting an IPv4 Network
Video Demonstration – Subnetting with the Magic Number (Cont.)
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Subnetting an IPv4 Network
Video Demonstration – Subnetting with the Magic Number (Cont.)
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Subnetting an IPv4 Network
Classless Subnetting Example
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Subnetting an IPv4 Network
Creating 2 Subnets
/25 Subnetting Topology
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Subnetting an IPv4 Network
Video Demonstration – Creating Two Equal-sized Subnets (/25)
Create 2 Equal-sized Subnets from 192.168.1.0 /24
Subnet Mask - 11111111.11111111.11111111.10000000
Magic Number = 128
192.168.1.0 /25 (start at 0)
192.168.1.128 /25 (Add 128)
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Subnetting an IPv4 Network
Subnetting Formulas
Calculate Number of Subnetting a /24 Network
Subnets Formula
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Subnetting an IPv4 Network
Subnetting Formulas (Cont.)
Calculate Number of
Calculating the Number of Hosts
Hosts Formula
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Subnetting an IPv4 Network
Creating 4 Subnets
/26 Subnetting Topology
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Subnetting an IPv4 Network
Creating 4 Subnets (Cont.)
/26 Subnetting Topology
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Subnetting an IPv4 Network
Creating 4 Subnets (Cont.)
/26 Subnetting Topology
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Subnetting an IPv4 Network
Video Demonstration – Creating Four Equal-sized Subnets (/26)
Create 4 Equal-sized Subnets from 192.168.1.0 /24
Subnet Mask in Binary – 11111111.11111111.11111111.11000000
2^2 = 4 Subnets
Magic Number = 64
192.168.1.0 /26
192.168.1.64 /26
192.168.1.128 /26
192.168.1.192 /26
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Subnetting an IPv4 Network
Video Demonstration – Creating Eight Equal-sized Subnets (/27)
Create 8 Equal-sized Subnets from 192.168.1.0 /24
Borrow 3 bits – 11111111.11111111.11111111.11100000
Magic Number = 32
192.168.1.0 /27 (Start at 0)
192.168.1.32 /27 (Add 32 to previous network)
192.168.1.64 /27 (Add 32)
192.168.1.96 /27 (Add 32)
192.168.1.128 /27 (Add 32)
192.168.1.160 /27 (Add 32)
192.168.1.192 /27 (Add 32)
192.168.1.224 /27 (Add 32)
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Subnetting a /16 and /8 Prefix
Creating Subnets with a /16 prefix
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Subnetting a /16 and /8 Prefix
Creating 100 Subnets with a /16 prefix
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Subnetting a /16 and /8 Prefix
Calculating the Hosts
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Subnetting a /16 and /8 Prefix
Video Demonstration – Creating One Hundred Equal-sized Subnets
An enterprise network requires 100 equal-sized subnets
starting from 172.16.0.0/16
• New Subnet Mask
• 11111111.11111111.11111110.00000000
• 2^7 = 128 Subnets
• 2^9 = 512 hosts per subnet
• Magic Number = 2
• 172.16.0.0 /23
• 172.16.2.0 /23
• 172.16.4.0 /23
• 172.16.6.0 /23
• …
• 172.16.254.0 /23
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Subnetting a /16 and /8 Prefix
Creating 1000 Subnets with a /8 Network
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Subnetting a /16 and /8 Prefix
Creating 1000 Subnets with a /8 Network (Cont.)
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Subnetting a /16 and /8 Prefix
Video Demonstration – Subnetting Across Multiple Octets
New Challenge Problem: Create
over 300 Equal-sized Subnets of
20,000 Hosts each starting from
10.0.0.0/8
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Subnetting to Meet Requirements
Subnetting Based on Host Requirements
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Subnetting to Meet Requirements
Subnetting Based On Network Requirements
Host devices
used by
employees in
the Engineering
department in
one network
and
Management in
a separate
network.
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Subnetting to Meet Requirements
Network Requirement Example
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Subnetting to Meet Requirements
Network Requirement Example (Cont.)
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Benefits of Variable Length Subnet Masking
Traditional Subnetting Wastes Addresses
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Benefits of Variable Length Subnet Masking
Variable Length Subnet Masks (VLSM)
Traditional Subnets of Varying Sizes
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Benefits of Variable Length Subnet Masking
Basic VLSM
Basic Subnetting
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Benefits of Variable Length Subnet Masking
Video Demonstration – VLSM Basics
Basic VLSM
• Subnets do not have to be equal sizes, as long
as their address ranges do not overlap.
• When creating subnets it is easier to work from
larger to smaller.
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Benefits of Variable Length Subnet Masking
VLSM in Practice
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Benefits of Variable Length Subnet Masking
VLSM Chart
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Benefits of Variable Length Subnet Masking
Video Demonstration – VLSM Example
Given the network 172.16.0.0
/23 = 2^9 hosts = 512
/23 creates subnets: 256+128+64+32+16+16 = 512 hosts needed
• 1 network for 200 hosts - 256 Address range 172.16.0.0 – 172.16.1.255
• 1 network for 100 hosts - 128
• 1 network for 50 hosts - 64
• 1 network for 25 hosts - 32
• 1 network for 10 hosts - 16
• 4 point-to-point networks for 2
hosts each – 4x4 = 16
172.16.1.248 /30 (4)
172.16.1.252 /30 (4)
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8.2 Addressing Schemes
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Structured Design
Network Address Planning
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Structured Design
Planning to Address the Network
Each host in an internetwork must have
a unique address.
Need proper planning & documentation.
Must provide & control access to servers
from internal hosts and external hosts.
Layer 3 STATIC address assigned to a
server can be used to control access to
that server.
Monitoring security and performance of
hosts means network traffic is examined
for source IP addresses that are
generating or receiving excessive
packets.
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Structured Design
Assigning Addresses to Devices
Devices that require addresses:
• End user clients
• Can be set for DHCP to save time and
manual errors.
• A change in the subnetting scheme requires
reconfiguration of DHCP server. IPv6 clients
use DHCPv6/SLAAC.
• Servers
• Configured with static addresses.
• Private addresses translated to public
addresses if accessible from the Internet.
• Intermediary devices
• Set with static addresses for remote
management.
• Gateway
• Router interface used to exit the network.
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8.3 Design Considerations for IPv6
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Subnetting an IPv6 Network
The IPv6 Global Unicast Address
IPv6 subnetting is not concerned
with conserving address space. Structure
IPv6 subnetting is about building an
addressing hierarchy based on the
number of subnetworks needed.
IPv6 link-local address is never
subnetted.
IPv6 global unicast address can be
subnetted.
IPv6 global unicast address
normally consists of a /48 global
routing prefix, a 16 bit subnet ID,
and a 64 bit interface ID.
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Subnetting an IPv6 Network
Subnetting Using the Subnet ID
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Subnetting an IPv6 Network
IPv6 Subnet Allocation
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8.4 Chapter Summary
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Conclusion
Chapter 8: Subnetting IP Networks
Implement an IPv4 addressing scheme to enable end-to-end connectivity in a small to medium-
sized business network.
Given a set of requirements, implement a VLSM addressing scheme to provide connectivity to end
users in a small to medium-sized network.
Explain design considerations for implementing IPv6 in a business network.
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