Network Addressing
Structure
CCNA Discovery2: Chapter 4
Version 4.1
Contents
 4.1: IP Addressing & Subnetting Review
 4.2: VLSM & CIDR
 4.3: NAT and PAT
IP Addresses
 IP addresses identify hosts and network devices
 To send and receive messages on an IP network,
every host must be assigned a unique 32-bit IP
address
 IP address are displayed in dotted-decimal
notation
 192.168.1.1
 Each of the 4 octets represents 8 bits
 IP addresses are hierarchical
 The network portion identifies the network that
a host belongs to
 The host portion identifies an individual host on
a network
Network Addresses
 The network portion of the address, is used to
represent the entire network
 It represents a group of IP addresses that can be
used on that network
 The network address consists of the network
field plus all 0’s in the host portion of the
address
 192.168.18.00000000
 192.168.18.0
 The Network address is not a usable host IP
address
 Network addresses are only used by routers to
decide how to get packets to their destination
Host vs. Network Portion
Network Number
Host Number
Broadcast Address
 A Broadcast Address is the address used to
send messages to every host on the same
network
 A Broadcast Address consists of the
Network address, plus all 1’s in the host
field
 The Broadcast address is NOT a USABLE
host address and can not be assigned to a
host
Broadcast Addresses
Network Address
 120.0.0.0
 170.50.0.0.
 192.168.10
Broadcast Address
120.255.255.255
170.5.255.255
192.168.10.255
Usable Host Addresses
 As we just saw, the Network address and the
Broadcast address are NOT usable host
addresses
 A usable host IP address is an IP address that:
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Is not a Network Address (all 0’s in host field)
Is not a Broadcast Address (all 1’s in host field)
Is not a reserved Address (127 addresses)
Is a Class A, B or C address
 Only a usable host IP address can be assigned
to a host device
Determining Usable Host
Addresses
Network
Usable Hosts
Broadcast
10.0.0.0
10.0.0.1 – 10.255.255.254
10.255.255.255
172.16.0.0
172.16.0.1-172.16.255.254
172.16.255.255
192.168.1.0
192.168.1.1-192.168.1.1254
192.168.1.255
Available Host Addresses
 The number of available host addresses on a network
can be calculated with the formula:
2 ^ host bits – 2
Network type
Available Hosts
 255.0.0.0
2 ^ 24 -2 =
16, 277, 214
 255.255.0.0
2 ^ 16- 2 =
65, 534
 255.255.255.0 2 ^ 8 – 2 =
254
 The reason we always subtract 2 from the total host
addresses to determine the available host addresses,
is because the network address and broadcast address
are NOT usable host address
 Therefore, every network has 2 addresses that can not
be assigned to hosts, the very 1st address (all 0’s in the
host portion) and the very last address (all 1’s in the
host portion)
IP Address Classes
 To create more possible network designations,
the 32-bit address space was organized into five
classes.
 Class A, B, and C: Commercial networks
 Class D and E: multicast and experimental
 The class of a network is indicated by the
values of the first few bits of the IP address,
called the high-order bits.
IP Address Classes
 Early Networks were only identified with an 8 bit
network address
 To create more possible network designations,
the 32-bit address space was organized into five
classes.
 Class A, B, and C: Commercial networks
 Class D and E: multicast and experimental
 Routers needed to be programmed to look
beyond the first 8 bits to identify class B and
C networks.
 Networks were divided in a way that makes
it easy for routers and hosts to determine
the correct number of network ID bits
Commercial IP Address Classes
 Class C addresses are usually assigned to small
networks
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Use 3 octets for the network and 1 for the hosts
N.N.N.H
The default subnet mask is 24 bits: 255.255.255.0
2, 097, 150 (2 ^ 21 – 2) possible networks
254 (2 ^ 8 – 2) available host addresses per network
 Class B addresses are typically used for medium-sized
networks
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Use 2 octets for the network and 2 for the hosts
N.N.H.H
The default subnet mask is 16 bits: 255.255.0.0
16, 382 (2 ^ 14 – 2) possible networks
65, 534 (2 ^ 16 – 2) available host addresses per network
 Class A addresses are typically assigned to large
organizations.
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Use 1 octet for the network and 3 for the hosts
N.H.H.H
The default subnet mask is 8 bits: 255.0.0.0
126 (2 ^ 7 – 2) possible networks
16, 777, 214 (2 ^ 24 – 2) available host addresses per network
Class A
 The first bit is always 0
 Addresses start with 0 to 126
Class B
 First two bits are always 1 and 0
 Addresses start with 128 to 191
Class C
 First three bits are always 1, 1 and 0
 Addresses start with 192 to 223
Class D
Class E
1 to 126
Private IP Addresses
 Reserved address space for private networks
 Private IPs are not routable on the Internet
 Many networking devices give out private IPs
through DHCP
The Loopback Address
 There are also private addresses that can
be used for the diagnostic testing of
devices.
 This type of private address is known as
a loopback address.
 The class A, 127.0.0.0 network address,
is reserved for loopback testing.
 The loopback IP address, 127.0.0.1 is
used to test a NIC card to verify that it is
sending and receiving signals.
Subnet Masks
 A subnet mask is a 32 bit address which tells
devices which part of the IP address is network
and which part is host
 Let routers & hosts figure out which network or
subnet an IP address belongs to
 Subnet Masks contain:
 all 1’s in the network field
 all 0’s in the host field
 Example Subnet Masks:
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255.255.255.0
255.255.0.0
255.255.255.128
255.254.0.0
Subnet Mask Formats
Subnet Masks can be written in 2 different
formats:
1. Dotted Decimal format
 192.168.1.1 255.255.255.0
2. Bit-Mask Format
 192.168.1.1 /24
 This indicates that there are 24 bits ( 24 1’s) in
the network and subnetwork portion of the
address (255.255.255.0)
4.2: Types of Subnetting
The Need for Subnetting
 Networks continued to grow and connect to
the Internet throughout the 80s and 90s,
with many organizations adding hundreds,
and thousands of hosts to their network.
 This created 3 needs or problems:
 The need to create separate LANS within a
company for security or management purposes.
 Increased hosts increased the broadcast traffic
which decreased network performance
 There are a limited number of Class B and C
addresses available
Example Scenario
 An ISP customer has outgrown its initial network
installation - the original integrated wireless router
is overloaded with traffic from both wired and
wireless users
 They have a Class C network address
 Solution:
 Add a 2nd networking device (larger integrated service
router)
 When adding a device, it is a good practice to place the
wired and wireless users on separate local subnetworks
to increase security
 The new network configuration requires that the
existing Class C network be divided into at least three
subnetworks
Example Scenario
Subnet 3
Subnet 2
Subnet 1
Subnets Defined
 RFC 917 defines Internet Subnets
 The Subnet mask is the method routers use to
isolate the network portion from an IP address.
 Routers read subnet masks left to right, bit
for bit
 Bits set to 1 are read as part of the network ID
 Bits set to 0 are read as part of the host ID
Altering the Address Hierarchy
 In the original IP address hierarchy, there
are 2 levels:
 Network field (network bits)
 Host field (host bits)
 Subdividing a classful network adds a new
level to the network hierarchy
 It creates 3 levels of Hierarchy in a IP
Address:
 Network (network bits)
 Subnetwork (subnet bits)
 Host (host bits)
Classful Subnetting
 Traditional classful subnetting has these
characteristics:
1.
2.
3.
4.
Uses a fixed number of subnets
Has a fixed number of hosts per subnet
All subnets must be the same size
Each subnet must use the same subnet mask
 Also known as fixed-length subnetting
 All subnets must be the same size, which means
that the maximum number of hosts that each
subnet can support is the same for all subnets
created

The more bits that are taken for the subnet ID, the
fewer bits left for host IDs
Limits of Classful Subnetting
 The original classful subnetting method required
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that all subnets of a single classed network be the
same size.
This was because routers did not include subnet
mask information in their routing updates
A router programmed with 1 subnet address and
mask on an interface automatically applied that
same mask to the other network subnets in its
routing table.
This limitation required the use of fixed-length
subnets and subnet masks
This technique wastes a significant number of IP
addresses.
Example: Classful Subnetting
Network: 192.168.1.0 /24
Subnet 3: 2 hosts
172.16.1.96 /27
Subnet 2: 10 hosts
172.16.1.64 /27
Subnet 1: 30 hosts
192.168.1.32 /27
Example: Classful Subnetting
 Original Network address: 192.168.1.0 /24
 Subnet 1 needs 30 hosts so subnets will have to
be created that support at least 30 hosts
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3 bits are borrowed = 255.255.255.224 mask
5 host bits are left unborrowed
This provides 30 addresses per subnet
Subnet Addresses are:
 192.168.1.32
 192.168.1.64
 192.168.1.96
 This wastes many addresses in Subnet 2 and 3
VLSM
 Variable length subnet masking (VLSM) helps
solve the limits of classful subnettting
 VLSM allows an address space to be divided into
subnets of various sizes
 This is done by subnetting subnets
 Characteristics of VLSM
 Each subnet can be a different size
 Each subnet can be designed to support the
number of hosts needed
 Each subnet can have a different subnet mask
How does it work
 In order for VLSM to work, Routers must be
aware of how the network was subnetted.
 With classful subnetting, we know that the
Subnet Mask information was not shared
with other routers
 With VLSM, routers must share subnet
mask information, so routers will know how
many bits have been used for the network
portion of each subnet address
 VLSM saves thousands of IP addresses
that would be wasted with traditional
classfull subnetting
Example: VLSM
Network: 192.168.1.0 /24
Subnet 3: 2 hosts
192.168.1.80 /30
Subnet 2: 10 hosts
192.168.1.64 /28
Subnet 1: 30 hosts
192.168.1.32 /27
Example: VLSM
 Original Network Address: 192.168.1.0 /24
 Subnet 1 needs 30 hosts:
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Need 30 hosts, so 5 bits must be left in the host portion
Borrow 3 bits = 255.255.255.224 mask
Subnet Address: 192.168.1.32 /27
This provides 30 addresses per subnet
 Subnet 2 needs 10 hosts
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Take the next available Subnet : 192.168.1.64
Need 10 hosts, so 4 host bits must be left over
Borrow 4 bits
Subnet mask = 255.255.255.240
Subnet Address: 192.168.1.64 /28
 Subnet 3 needs 2 hosts
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Take the next available subnet: 192.168.1.80
Need 2 hosts, so 2 host bits must be left over
Borrow 6 bits
Subnet mask = 255.255.255.252
Subnet Address: 192.168.1.80 /30
CIDR
 CIDR = Classless Inter-Domain Routing
 CIDR is a type of network addressing that ignores
the traditional network classes (Class A, B and C)
 CIDR Assigns Blocks of Addresses, based on the
number of hosts needed
 Can be though of as assigning a Subnet of a Class A or
Class B address to a company as a block of Addresses
 It identifies networks based solely on the number
of bits in the network prefix
 Example: 172.16.64.0 / 18
 /18 bits in the network portion of the address
 This block contains the Addresses: 172.16.64.1 to
172.16.127.255
CIDR
 CIDR protocols freed routers from using only the highorder bits to determine the network prefix
 registered IP addresses do NOT need to be assigned by class
 Before CIDR, an ISP requiring 3,000 host addresses could
request either a full Class B address space or multiple
Class C network addresses to meet its requirements.
 With a Class B address space, the ISP would waste thousands of
registered addresses.
 With multiple Class C addresses, it could be difficult to design the
ISP network so that no single section required more than 254 host
addresses.
 By ignoring the traditional address classes, CIDR enables
ISPs to request a block of addresses based on the number
of host addresses it requires.
 CIDR is defined in RFC 1519
Supernets
 Supernets are created by combining a group of
Class C addresses into one large block
 This enables addresses to be assigned more
efficiently
 Example: 192.168.0.0/19
 19 bits are used for the network prefix
 This block contains the addresses 192.168.32.1 to
192.168.63.255
 This allows 8,190 possible host addresses (213)
 An ISP can use the supernet as one large network
or divide it into as many smaller networks as
needed to meet its requirements.
Why learn classed addressing?
 Although classed addressing and fixedlength subnet masking are becoming less
common, it is important to understand how
these addressing methods work.
 Many networking devices still use the
default subnet mask if no custom subnet
mask is specified.
Router Interface Addressing

Each subnet is a separate network and a Router is
needed to communicate between Subnets
Every Router Interface must have a valid host IP
Address: this includes both WAN and LAN interfaces
WAN Interfaces: when 2 routers are connected,
there must be a separate network, or subnet
assigned to the connection between them
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LAN Interfaces: Each router interface connected to
a LAN must have an IP address in the same subnet
as the LAN
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The interfaces on both routers must be assigned host IP
addresses in that network or subnet
Each router interface is the default gateway for its subnet
Usually, router interfaces are assigned either the first
or last host address available in the subnet. This
assures consistency.
Communicate between Subnets
Subnet 3
WAN Interfaces
LAN Interfaces
Subnet 1
Subnet 2
4.3: NAT
 Network Address Translation
 NAT allows a group of private users to
access the Internet by sharing one or more
public IP addresses
 NAT translates private IP addresses into 1
or more public IP addresses for routing on
the Internet
NAT Advantages
 NAT has several advantages:
1. Saves registered IP addresses
 IP addresses can be re-used and many hosts on a
single LAN can share globally unique IP addresses
2. Increased security by
 Withholds hosts actual IP host addresses from
direct Internet access
3. Transparent to end users
4. Adds Scalability to LAN
NAT Disadvantages
1. Incompatible with certain applications
2. Prevents legitimate remote access to
network
3. Requires increased processing by router
which negatively affects network
performance
NAT Analogy
 As a company adds employees, at some point,
they no longer run a public phone line directly to
each employee desk.
 Instead, they use a system that allows the
company to assign each employee an extension
number.
 The company can do this because not all
employees use the phone at the same time.
 Using private extension numbers enables the
company to purchase a smaller number of
external phone lines from the phone company.
NAT at Work
Inside vs. Outside Network
 Inside local network
 A network that is part of
the privately addressed
LAN
 Outside global network
 A network that is
external to the LAN and
does not recognize the
private addresses
assigned to hosts on
the LAN
Inside & Outside Addresses
 Inside local address
 A Private IP address configured on a host on an inside
network
 Must be translated before it can travel outside the local
network addressing structure
 Inside global address
 The NAT translated IP address
 The IP address of an inside host as it appears to the
outside network
 Outside local address
 The Destination address of the packet while it is on the
local network
 Usually, this is the same as the outside global address.
 Outside global address
 The Public IP address of an external host
Inside & Outside Addresses
Inside Global Address = NAT
Translated Public IP Address
Dynamic NAT
 Dynamic NAT dynamically translates each
inside local addresses to an inside global
address by using 1 public IP address, or a
pool of addresses
Static NAT
 What if one or more of the hosts within a network
are running services that need to be accessed
from the Internet?
 Static NAT translates a permanent registered
global address to particular hosts
 Static NAT is used for Servers that need a consistent IP
address
 Static translations ensure that an individual host private
IP address is always translated to the same registered
global IP address
 Static NAT allows hosts on the public network to access
selected hosts on a private network
PAT
 PAT (Port Address Translation) translates
multiple inside local addresses to a single global
address using Port numbers
 PAT is also called NAT overload
 PAT translates every inside local address to the
same inside global address, by using PORT
NUMBERS to represent the different private
internal addresses
 When a source host sends a message to a
destination host, it uses an IP address and port
number combination to keep track of each individual
conversation with the destination host
How PAT works
 PAT translates the local source address
and port combination in an outgoing packet
to a single global IP address and a unique
port number above 1024
 Each host is translated into the same global IP
address, but the port number associated with
the conversation is unique.
 Responding traffic is addressed to the
translated IP address and port number used
by the host.
 A table in the router contains a list of the
inside Local addresses and port numbers
PAT
PAT Security
 PAT conversations use a unique and combination
of the private IP address and port number
 Example: 192.168.1.106: 7000
 Uses Port numbers above 1024
 PAT Maximizes security
 Each private IP address/port number translation is ONLY
created when a host on the inside network initiates
communication
 The translation is only in place for the duration of the
connection, so a given user does not keep the same
global IP address and port number combination after the
conversation ends.
 Users on the outside network cannot reliably initiate
a connection to a host on a network that uses PAT.
IP Nat issues
1. Requires additional network workload to
support IP addresses and port translations
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
Some applications embed an IP address as part of
the encapsulated data
The router must replace the source IP addresses
and port in the data, and the source addresses in
the IP header.
2. Requires careful network design and
equipment selection

Routers must support PAT
3. Requires accurate configuration
IPv.6
 3 Solutions were developed to provide
to temporarily alleviate the problem of
IPv4 address depletion:
1. Subnetting
2. Private IP addressing
3. NAT / PAT
 IPv6 was proposed as a permanent
solution to the problem of IPv4 address
depletion
 Outlined in 1998 in RFC 2460
 The transition to IPv6 is ongoing
IPv6
 Uses a 128 bit Address
 Represented as 32 hexadecimal digits
separated by colons (
 8 groups of 4
 Ex: 2001:0db8:3c55:0015:0000:0000:abcd:ff13
 Uses a 3-part hierarchy:
 Global Prefix: assigned to an organization by an
Internet names registry
 12 Hex digits
 Subnet: identifies the Subnet
 4 Hex digits
 Interface Identifier: identifies the host
 16 Hex digits
IPv6 Address
IPv6 Improvements
 IPv6 offers many improvement over IPv4:
1.
2.
3.
4.
5.
Allows for more address space
Creates better space management
Allows easier TCP/IP administration
Incorporates modern Routing capabilities
Provides support for advanced network
capabilities
Summary
 Devices that want to communicate over a network need a unique IP

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address
IP addressing can be tailored to the needs of the network design
through the use of custom subnet masks.
A network can be divided into subnets to provide security and
preserve addresses
Subnets and custom subnet masks can be created by extending the
number of bits used for the network portion of the address
Communication between subnets requires a router
Classful subnetting uses the same subnet mask for each subnet
Classless subnetting gives classful IP addressing schemes more
flexibility through the use of variable length subnet masks.
Network Address Translation (NAT) allows a group of private IP
addresses to share a small pool of public IP addresses
Port Address Translation (PAT) translates multiple local addresses to
a single global IP address, maximizing the use of both private and
public IP addresses.
IPv6 offers improvements over IPv4