IPv4 Addressing

Binary Math

A bit — a zero or a one

• One digit. Off or on. Cold or hot. 0 or 1.

A byte — Eight bits

• Often called an “octet” to avoid ambiguity

A binary-to-decimal conversion chart

Binary to Decimal

What is binary 00000010 in decimal?

What is binary 10000010 in decimal?

What is binary 11111111 in decimal?

Decimal to Binary Conversion

What is decimal 154 in binary?

More bits, more addresses

More bits, more addresses:

Power of two

• Useful for binary calculations and subnetting

IPv4 Addressing

Networking with IPv4

IP Address, e.g., 192.168.1.165

• Every device needs a unique IP address

Subnet mask, e.g., 255.255.255.0

• Used by the local device to determine what subnet it’s on
  • The subnet mask isn’t (usually) transmitted across the network
  • You will ask for the subnet mask all the time

Default gateway, e.g., 192.168.1.1

• The router that allows you to communicate outside your local subnet
• The default gateway must be an IP address on the local subnet

Special IPv4 Addresses

Loopback address

• An address to yourself
• Ranges from 127.0.0.1 through 127.255.255.254
• An easy way to self-reference (ping 127.0.0.1)

Reserved addresses

• Set aside for future use or testing
• 240.0.0.1 through 254.255.255.254
• All “Class E” addresses

Virtual IP addresses (VIP)

• Not associated with a physical network adapter
• Virtual machine, internal router address

IPv4 addresses

Internet Protocol version 4

• OSI Layer 3 address

Since one byte is 8 bits, the maximum decimal value for each byte is 255

DHCP

IPv4 address configuration used to be a manual process

• IP address, subnet mask, gateway, DNS servers, NTP servers, etc.

Dynamic Host Configuration Protocol

• Provides automatic address and IP configuration for almost all devices

Automatic Private IP Addressing (APIPA)

A link-local address

• Can only communicate to other local devices
• No forwarding by routers

IETF has reserved 169.254.0.1 through 169.254.255.254

• First and last 256 addresses are reserved
• Functional block of 169.254.1.0 through 169.254.1.0

Automatically assigned

• Uses ARP to confirm the address isn’t currently in use

The IPv4 address problem

There are far more devices than IPv4 addresses

• This Internet thing could be big

The use and registration of IP address ranges is problematic

• Unused and non-continuous address blocks
• Complete depletion of available addresses

Private IP address ranges

More public IP addresses

• More Internet connectivity

Huge private IP address ranges

• Properly design and scale large networks

Private IP addresses are not Internet-routable

• But can be routed internally
• Use NAT for everything else

Defined in RFC 1918

• Request for Comment

Public addresses vs. Private addresses

RFC 1918 private IPv4 addresses

Classful Subnetting

Very specific subnetting architecture

• Not used since 1993
  • But still referenced in casual conversation

Used as a starting point when subnetting

• Standard values

Subnet Classes

The 127.0.0.0/8 network is reserved as a loopback address.

What IP class?

The Construction of a Subnet

Network address

• The first IP address of a subnet
• Set all host bits to 0 (0 decimal)

First usable host address

• One number higher than the network address

Network broadcast address

• The last IP address of a subnet
• Set all hosts bits to 1 (255 decimal)

Last usable host address

• One number lower than the broadcast address

Subnet calculations

IP address: 10.74.222.11

• Class A
• Subnet mask 255.0.0.0

IP address: 172.16.88.200

• Class B
• Subnet mask 255.255.0.0

IP address: 192.168.4.77

• Class C
• Subnet mask 255.255.255.0

IPv4 Subnet Masks

Classless Subnetting

CIDR (Classless Inter-Domain Routing)

• Created around 1993
• Removed the restrictions created by classful subnet masks
• “Cider” block notation

Subnet masks can be expressed as decimal or in CIDR notation

• IP address, slash, number of subnet bits; 192.168.1.44/24

You will usually be provided an IP address, subnet mask, default gateway, and DNS servers

• Some OSes are expecting decimal masks
• Some OSes are expecting CIDR notation masks

The subnet mask

Contiguous series of ones

• Ones on the left
• Zeros on the right

Binary to CIDR-block notation

Subnet Masks — Binary to Decimal

Now we can calculate binary to CIDR-block notation:

11111111.11110000.00000000.00000000
255     .   240  .    0   .    0
                /12

Network = 12 bits, Host = 20 bits

11111111.11111111.11100000.0000000
255     .    255 .    224 .   0
                /19

Network = 19 bits, Host = 13 bits

It can be done in the reverse too. From CIDR-block notion to decimal.

                /26
11111111.11111111.11111111.11000000
255     .   255  .   255  .    192

Network = 26 bits, Host = 6 bits

                /20
11111111.11111111.11110000.00000000
255     .   255  .   240  .    0

Network = 20 bits, Host = 12 bits

Calculating IPv4 Subnets and Hosts

Why subnet the network?

• For single device, it’s functionally impossible to know about all devices on the big network
• That’s why we create smaller networks using subnets

VLSM (Variable Length Subnet Masks)

Class-based networks are inefficient

• The subnet mask is based on the network class

Allow network administrators to define their own masks

• Customize the subnet mask to specific network requirements

Use different subnet masks in the same classful network

• 10.0.0.0/8 is the class A network
• 10.0.0.1/24 and 10.0.8.0/26 would be VLSM

Defining subnets

IP address: 10.0.0.0

• Class A, subnet mask: 255.0.0.0

Calculating subnets and hosts

Powers of two

Number of Subnets = 2^{subnet bits}

Hosts per subnet = 2^{host bits} — 2

Let’s put the table to use:

IP address: 10.1.1.0/24

• Class A
• Subnet mask 255.0.0.0
• /24 in binary = 11111111.11111111.11111111.00000000

Total Subnets = 16 bits = 2¹⁶ = 65,536

Hosts per Subnet = 8 Bits = 2⁸ = 256-2 = 254

IP address: 192.168.11.0/26

• Class C
• Subnet mask 255.255.255.0
• /26 in binary = 11111111.11111111.11111111.11000000

Total Subnets = 2 bits = 2² = 4

Hosts per subnet = 6 bits = 2⁶ 2 = 64–2 = 62

IP address: 172.16.55.0/21

• Class B
• Subnet mask 255.255.0.0
• /21 in binary = 11111111.11111111.11111000.00000000

Total subnets = 5 bits = 2⁵ = 32

Total hosts per subnet = 11 bits = 2¹¹–2 = 2048-2 = 2046

Magic Number Subnetting

Subnetting the network

Here’s your IP address assignment: Network: 192.168.1.0/24

We need an IP addressing scheme with more than one network address that can support 40 devices per subnet.

Calculating subnet masks:

• We have four networks with about 40 devices per subnet

Subnetting the network:

IP address 192.168.1.0, subnet mask 255.255.255.192

192.168.1.0 = 11000000.10101000.00000001.00000000

255.255.255.192 = 11111111.11111111.11111111.11000000

Network = 24 bits, Subnet = 2, Host = 6

Total Subnets = 2 bits = 2² = 4

Hosts per Subnet = 6 bits = 2⁶ = 64-2 = 62

Four important addresses

Network address/subnet ID

• The first address in the subnet

Broadcast address

• The last address in the subnet

First available host address

• One more than the network address

Last available host address

• One less than the broadcast address

Subnetting the network

Magic Number subnetting

Very straightforward method

• Can often perform the math in your head

Subnet with minimal math

• Still some counting involved

Some charts might help

• But may not be required
• CIDR to Decimal
• Host ranges

Some helpful charts

CIDR to decimal charts

• Memorization will increase the speed

More simple chart:

Magic number chart:

Host ranges

• Larger blocks are easier to remember
• Multiply quickly for the smaller blocks

The magic number process

• Convert the subnet mask to decimal (if necessary)
• Identify the “interesting octet”
• Calculate the “magic number”
  • 256 minus the interesting octet
• Calculate the host range
• Identify the network address
  • First address in the range
• Identify the broadcast address
  • Last address in the range

Find the subnet ID

IP address: 165.245.77.14 Subnet mask: 255.255.240.0

• If the mask is 255, Copy the IP address
• If the mask is zero, copy the zero
• Anything not 255 or zero is the interesting octet

Subtract the interesting octet mask from 256

• 256-240 = 16
• The magic number is 16

To find out Subnet ID, see the table.

Find the broadcast address

• If the mask is 255, copy the subnet ID
• If the mask is zero, write 255
• Anything not 255 or zero is the interesting octet

Subtract the interesting octet mask from 256

• 256-240 = 16
• The magic number is 16

Broadcast ID = (Calculate Subnet ID + Magic Number) — 1

= (64+16)-1 = 79 (Broadcast ID)

Find the host range

IP address: 165.245.77.14

Subnet mask: 255.255.240.0

Subnet ID: 165.245.64.0

Broadcast: 165.245.79.255

Firt host is subnet ID + 1

• 165.245.64.1

Last host is broadcast - 1

• 165.245.79.254

Finding the Subnet ID/Broadcast and First and Last host addresses

IP address: 10.180.122.244 Subnet mask: 255.248.0.0

Find the Subnet ID:

Using the Subnet ID rules

Mask          255.    248.     0.    0
Action        copy  (256-248) Zero  Zero
IP             10.     180.  122.  244
Subnet ID      10.     176.    0.    0

Find the Broadcast ID:

Using the Broadcast ID rules

Broadcast ID = (Subnet ID + Magic Number) — 1 = (176+8)-1 = 183

Mask           255.    248.    0.    0
Action        copy  (256-248)  255  Zero
Subnet ID       10.    176.    0.    0
Broadcast ID    10.    183.  255.    255

So,

IP address: 10.180.122.244

Subnet mask: 255.248.0.0

Subnet ID: 10.176.0.0

Broadcast address: 10.183.255.255

First host address: 10.176.0.1

Last host address: 10.183.255.254

Speeding up the magic

IP address: 172.16.242.133/27

Using the table, /27 has subnet mask

Subnet mask: 255.255.255.224

Magic number is 256–224 = 32

From the host ranges, the Subnet ID would be

Subnet ID: 172.16.242.128

Using the magic number calculation: (subnet ID + magic number) — 1

Broadcast: 172.16.242.159

First host: 172.16.242.129

Last host: 172.16.242.158

Seven Second Subnetting

Designed for exam situations

• Very fast subnetting
• No second guessing

No Very little math involved

• Some simple addition to create the tables
• Add and subtract one

Combination of many techniques

• Find one that works for you

Use the in-person or digital whiteboard

• Quickly create the charts — bring your own erasable marker

The Networks

The Hosts

Network Address Subnet Boundaries:

The Seven Second Subnetting Process

Convert IP address and subnet mask to decimal

• Use chart to convert between CIDR-block notation and decimal
• Same chart also shows the number of devices per subnet

Determine network/subnet address

• Second chart shows the starting subnet boundary

Determine broadcast address

• Second chart shows the ending subnet boundary

Calculate first and last usable IP address

• Add one from network address, subtract one from broadcast address

Seven second subnetting — Practice

Address: 165.245.12.88/24

Convert address and mask to decimal

Calculate the network address:

• If mask is 255, bring down the address
• If mask is 0, use the 0

Calculate the broadcast address:

• If the mask is 255, bring down the address
• If mask is 0, use 255

First IP is network address + 1 Last IP is broadcast address - 1

Address: 165.245.12.88/26

(For any other number, refer to your charts)

Address: 165.245.12.88/20

Address: 18.172.200.77/11

Address: 18.172.200.77/17

Subnetting and exams

Practice creating the charts

• Writing or typing
• Less than a minute

Consider using your own dry-erase pen

• Fine tip
• Get permission from the testing center

Find the system that works for you

• Magic number, seven second subnetting, or your own shortcuts