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Subnetting

Subnetting is the process of dividing a single network into smaller network segments, or subnets.
This is essential for efficient IP address management and improving network performance.

Table of Contents

About IPv4 Addresses

An IPv4 address is 32 bits long. These are divided into two parts: 1. Network bits.
2. Host bits.

Network Classes

First we determine what class the network is.

Class A IP addresses have the first bit of the first octet set to 0.
Range: 0.0.0.0 to 127.255.255.255
 Network Bits: 8

Class B IP addresses have the first two bits of the first octet set to 10.
Range: 128.0.0.0 to 191.255.255.255
 Network Bits: 16

Class C IP addresses have the first three bits of the first octet set to 110.
Range: 192.0.0.0 to 223.255.255.255
 Network Bits: 24

Class Network Bits Range First Bit(s) of the First Octet
Class A 8 0 - 127 0
Class B 16 128 - 191 10
Class C 24 192 - 223 110
Class D and E: Class D addresses are used for multicast groups and Class E addresses are reserved for experimental purposes. They are not typically used for general networking.

Subnet Mask

  • A subnet mask is used to determine which portion of an IP address refers
    to the network, and which part refers to hosts.
  • In CIDR notation, the subnet mask is denoted by a suffix (like /24), indicating the number of bits allocated for the network portion, which is the first number in the IPv4 address.
    • For each 8 network bits an IP has, one of the numbers is reserved for the
      network.

Host Bits

For example, in 192.168.1.0/24, only the last number can be used for hosts.
The first number (192 the first 8 network bits) is reserved for the network.
 The second number (168 the next 8 network bits) is reserved for the network.
The third number (1 the next 8 network bits) is reserved for the network.
 At the fourth number (0), all 24 network bits are accounted for, so
that number can be used for hosts (1-254).

More complex subnets can be made by "borrowing bits" from the network portion of the IP.
Those can be calculated by converting the decimal number to binary.

Binary

A base2 (binary) number has 2 states of nature (0-1)
A base10 number has 10 states of nature (0-9)

To covert other numbers to base10, go from right to left, starting from 0.
The right-most number will be exponent

States of Nature

Based on the number of bits, the amount of available numbers changes.

Number of Bits States of Nature
1 2
2 4
3 8
4 16
5 32
6 64
7 128
8 256
9 512
10 1024
11 2048
12 4096
13 8192
14 16384
15 32768
16 65536

Determining Network Class via Binary

Converting an IPv4 Address to Binary

Each set in the IPv4 address is usually displayed as base10.
To convert it into binary, we would need to exponent 2 (number of binary states), 8 times. In binary, 8 bits represent 256 states of nature. We'd cut that in half for each step.
For each of the bits for each number (8), we'd multiply it like so:

Exponent: 2⁷ 2⁶ 2⁵ 2⁴ 2³ 2² 2¹ 2⁰
States: 128's 64's 32's 16's 8's 4's 2's 1's

For each number in the IP (up to the host numbers):
if a 128 goes into it, subtract 128 and add that bit (1)
 if a 64 goes into it, subtract 64 and add that bit (1)
* Do the same for 32, 16, 8, 4, 2, and 1

Type First Set Second Set Third Set Fourth Set
base10 192 168 1 0
base2 ________. ________. ________. HHHHHHHH. 

192  - 128  =  64    :  1  (128 goes into 192: TRUE)  
64   -  64  =  0     :  1  ( 64 goes into the remainder: TRUE)  
0    -  32  =  0     :  0  ( 32 goes into the remainder: FALSE)  
0    -  16  =  0     :  0  ( 16 goes into the remainder: FALSE)  
0    -   8  =  0     :  0  ( 16 goes into the remainder: FALSE)  
0    -   4  =  0     :  0  ( 4  goes into the remainder: FALSE)  
0    -   2  =  0     :  0  ( 2  goes into the remainder: FALSE)  
0    -   1  =  0     :  0  ( 1  goes into the remainder: FALSE)
So the first set will look like:

Type First Set Second Set Third Set Fourth Set
base10 192 168 1 0
base2 11000000. ________. ________. HHHHHHHH.

Or we can use our exponent table (the number is 192):
| Exponent: | 2⁷ | 2⁶ | 2⁵ | 2⁴ | 2³ | 2²| 2¹ | 2⁰ | |---------------|------|------|------|-------|------|----|------|-------| | States: |128's | 64's | 32's | 16's | 8's | 4's | 2's | 1's | | Bit Repr: | _ | _ | _ | _ | _ | _ | _ | _ |

  1. Start with the highest bit value (128, or 2727) and see if it fits into 192.
    Since 128 is less than 192, it fits, so you set the first bit to 1.
    Then subtract 128 from 192, leaving 64.
Exponent 2⁷ (128) 2⁶ (64) 2⁵ (32) 2⁴ (16) 2³ (8) 2² (4) 2¹ (2) 2⁰ (1)
Result 192 - 128 = 64
Bits 1 _ _ _ _ _ _ _
  1. Next, check if 64 (2626) fits into the remainder (64). It does, so set the second bit to 1. Subtract 64 from 64, leaving 0.
Exponent 2⁷ (128) 2⁶ (64) 2⁵ (32) 2⁴ (16) 2³ (8) 2² (4) 2¹ (2) 2⁰ (1)
Result 64 - 64 = 0
Bits 1 1 _ _ _ _ _ _
  1. Since the remainder is now 0, all remaining bits are set to 0. So for the first octet of bits we have 11000000.

That leaves us with this:
| Type | First Set | Second Set | Third Set | Fourth Set | |--------|--------------|---------------|--------------|--------------| | base10 | 192 | 168 | 1 | 0 | | base2 | 11000000. | ________. | ________. | HHHHHHHH. |

There are two identifying factors for the IP's class:
The first number falls inside the range of 192 to 223.
This identifies it as a Class C IP Address.
 The first three bits in the first octet are 110.
This also identifies it as a Class C IP Address.

Our network is Class C, so it has 24 network bits.

Network Bits

This is the table from network classes

Class Network Bits Range First Bit(s) of the First Octet
Class A 8 0 - 127 0
Class B 16 128 - 191 10
Class C 24 192 - 223 110

The lower the number of network bits, the more HOST bits are available.

Each of the network bits refer to one of the bits in the binary representation of the IP.

For instance, if we have a "Class A" IP with 8 network bits, the network bits are reserved for the first "set" (the first binary octet).
For the IP 127.0.0.0:
| Type | First Set | Second Set | Third Set | Fourth Set | |--------|--------------|---------------|--------------|--------------| | base10 | 127 | 0 | 0 | 0 | |Network | NNNNNNNN. | HHHHHHHH. | HHHHHHHH. | HHHHHHHH. | | base2 | 01111111. | ________. | ________. | ________. |

The Ns represent the bits reserved for the network.
The rest (H) are available hosts.
The first bit is set to 0, so we know it is a Class A IP address.

However, if we have a "Class B" IP with 16 network bits, the network
will reserve the first two octets:
For the IP 191.0.0.0:
| Type | First Set | Second Set | Third Set | Fourth Set | |--------|--------------|---------------|--------------|--------------| | base10 | 191 | 0 | 0 | 0 | |Network | NNNNNNNN. | NNNNNNNN. | HHHHHHHH. | HHHHHHHH. | | base2 | 10111111. | ________. | ________. | ________. | The first two bits are 10, so we know it's a Class B IP address.

CIDR: Classless Inter-Domain Routing

  • CIDR uses a notation that includes the IP address followed by a slash, and then the number of bits in the subnet mask.
  • In CIDR notation, /24 or /30 indicate the number of bits used for the network portion of the address. This number is called the
    "Subnet Mask"

For example, 192.168.1.0/24 indicates that the first 24 bits of the IP address are used for network identification, and the remaining bits are for host identification.

VLSM: Variable-Length Subnet Masking

CIDR uses VLSM, which means the subnet mask length can vary (it's a variable).
This allows networks to be divided into subnetworks of different sizes, making address allocation more flexible.

The IP: 192.168.1.0

In these notes, "each number" will refer to 192, 168, and 1.
The last number (0) doesn't matter - those will be the hosts available on the subnets.
Each number is represented by an octet of binary bits (8 bits).

The IP now with network bits: 192.168.1.0/24

192.168.1.0/24
* The /24 means there are 24 network bits (Class C)
* This is referred to as the "CIDR" (Classless Inter-Domain Routing) notation.

Type First Set Second Set Third Set Fourth Set
base10 192 168 1 0
Network NNNNNNNN. NNNNNNNN. NNNNNNNN. HHHHHHHH.
base2 11000000. ________. ________. ________.

Where:
N = Network Bits
 H = Host Bits

In the binary representation (base2):
* The first three sets ("First Set", "Second Set", "Third Set") each consist of
8 bits (or "octets"), making up 24 bits in total, dedicated to the network portion (/24).

  • The "Fourth Set" represents the host portion, which consists of 8 bits, allowing
    for 256 different states (0 to 255).

In this instance:
192.168.1.0 refers to the network address.
 192.168.1.255 would be the broadcast address.
* 192.168.1.1 through 192.168.1.254 are valid host addresses.

The host numbers 0 and 255 are reserved:

  • 0 is reserved for the Network
  • 255 is reserved for the Broadcast
    192.168.1.0       = The network itself  
192.168.1.255     = The broadcast  
192.168.1.{1,254} = available as hosts
    So we have 1-254 available as hosts.
  1. Split it in half by adding ("borrowing") a bit

192.168.1.0/25
1-127 is the first subnet
 128-254 is the second subnet

  1. Split it in half again by borrowing another bit:

192.168.1.0/26
The subnets for this network are now:
1-63 * 64-127
 128-191
* 192-254

  1. Split it in half yet again by adding another bit (now 1/8th of the original size)

192.168.1.0/27
The subnets for the network now:
0-31
 32-63 * 64-95
96-127
 128-159
160-191
 192-223 * 224-255

000
001
010
011
100
101
110
111

Just count in 32s (bottom to top)
224- 192-
160-
128
96
64
32 0

Subnetting Formulas and Definitions (for reference):

Formula for the Number of Hosts

To determine the number of hosts in a subnet, use the formula:

2^(32 - subnet_mask) - 2
For example, in a /24 subnet (a subnet with 24 network bits), there
are 2^(32 - 24) - 2 = 254 usable host addresses.
2^(32 - 24) - 2 = 254

  • This formula calculates the number of usable hosts in a subnet.
  • H represents the number of bits used for the host portion of the subnet.
  • The -2 accounts for two addresses in every subnet that cannot be assigned to
    hosts:
    • the network address (all host bits are 0)
    • the broadcast address (all host bits are 1)

Forumla for the Number of Subnets

  • Subnets = 2^S
    • This is used to calculate the number of subnets created when you subnet an existing network.
    • S is the number of bits borrowed from the host part to create more networks.

Formula for the Subnet Mask

  • Masking: N=1's | S=1's | H = 0's
    • This represents the structure of a subnet mask.
      • The network (N) portions are represented by 1s
      • The subnet (S) portions are represented by 1s
      • The host (H) portion is represented by 0s

Steps to Calculate the Subnet Mask 1. Determine the number of bits used for the subnet mask (from the CIDR notation). 2. Set the corresponding number of bits to 1 (starting from the left), followed by 0s for the remaining bits. 3. Convert the binary subnet mask into decimal format for easy representation.

Checking for the Right Number of Bits

(( NETWORK_BITS + SUBNET_BITS + HOST_BITS = 32 ))
* N+S+H = 32 * This equation is specific to IPv4 addressing.
* N is the number of network bits
* S is the number of subnet bits
* H is the number of host bits
* Since IPv4 addresses are 32 bits in total, the sum of network, subnet, and host bits must equal 32.
* The 32 represents the four binary bit octets that make up the IP.

Network and Broadcast Addresses

  • N = all H = 0
    • This represents the network address of a subnet. All host bits (H) are set to 0.
  • B = all H = 1
    • This represents the broadcast address of a subnet. All host bits (H) are set to 1.

What do these formulas represent? They are related to subnetting.

Subnetting Terms:
CIDR - Classless Inter-Domain Routing
 VLSM - Variable-Length Subnet Masking

Routing Protocols:
OSPF - Open Shortest Path First
 BGP - Border Gateway Protocol

In the cloud

You can not go any lower than 16 network bits on AWS (possibly other VPCs too)
10.100.0.0/16 <-

What You Should Know About CIDR for Networking Proficiency

Understanding Subnetting

Proficiency in CIDR involves a strong grasp of subnetting concepts, including how to divide networks into subnets, calculate the number of available hosts, and understand the implications for network design and routing.

Subnetting is the process of dividing a single network into smaller
network segments, or subnets.
This is essential for efficient IP address management and improving
network performance.

  • Subnet Mask
    • A subnet mask is used to determine which portion of an IP address refers to the network and which part refers to hosts.
    • In CIDR, the subnet mask is denoted by a suffix (like /24), indicating
      the number of bits allocated for the network portion.
  • Host Calculation
    • To determine the number of hosts in a subnet, use the formula 2^(32 - subnet mask) - 2
      • For example, in a /24 subnet, there are 2^(32 - 24) - 2 = 254 usable host addresses.
  • Subnet Sizes
    • Understand how changing the subnet mask affects the size of a network.
      • A smaller subnet mask (like /23) results in fewer network bits and more host bits, thus a larger number of hosts.
  • Practice
    • Regularly practice subnetting exercises to become proficient in:
      • calculating network and broadcast addresses
      • calculating the number of hosts
      • subnet division

IP Address Planning

IP Address Planning is the skill of planning and allocating IP addresses efficiently for different sized networks, considering future growth and
network segmentation.

Efficient IP address planning is critical for network scalability and management.

  • Assessing Needs
    • Assess the needs of the network, including:
      • The number of required hosts
      • Network segments (subnets)
      • Potential future growth
  • Allocating Addresses
    • Allocate IP addresses based on the size of the network.
    • Use smaller subnets for networks with fewer hosts to conserve IP addresses.
  • Addressing Schemes
    • Develop a logical addressing scheme that simplifies management and allows
      for easy expansion.
    • Group similar devices or departments into the same subnet when possible.

Routing Protocols and CIDR

Understanding how modern routing protocols (like OSPF, BGP) interact with CIDR, including the concepts of route summarization and supernetting.

Modern routing protocols leverage CIDR for efficient routing.

  • Route Summarization
    • This involves combining multiple subnets into a single advertisement
      to reduce the size of routing tables.
      • For example, 192.168.0.0/24 and 192.168.1.0/24 can be summarized as 192.168.0.0/23.
  • Protocol Understanding
    • Familiarize yourself with protocols like OSPF (Open Shortest Path First), and BGP (Border Gateway Protocol), and how they use CIDR for route advertisement and path selection.

OSPF (Open Shortest Path First)
BGP (Border Gateway Protocol)

Practical Application

Practice with real-world scenarios, including setting up network environments with various subnet sizes, configuring routers, and understanding the impact on routing tables.

Hands-on experience is vital.

  • Network Setup
    • Set up different network scenarios using subnetting.
    • Implement these in lab environments or simulations.
  • Troubleshooting
    • Practice troubleshooting common network issues related to subnetting, such as IP conflicts or routing problems.

IPv6 and CIDR

Familiarity with CIDR in the context of IPv6, as IPv6 becomes more prevalent and requires a different approach due to its larger address space.

IPv6 uses CIDR as well, but with a much larger address space.

  • Address Structure
    • Understand the structure of IPv6 addresses and how they differ from IPv4.
  • Subnetting in IPv6
    • Learn how subnetting works in IPv6, where the subnet size is typically /64, but can vary.

Network Tools

Proficiency in using network tools for CIDR calculations, subnetting, and addressing. This includes both command-line tools and software applications.

Use tools to simplify CIDR-related tasks.

  • CIDR Calculators
    • Tools like subnet calculators can automate subnetting tasks.
  • Command-Line Tools
    • Get comfortable with command-line tools like ipcalc or ifconfig/ip for subnet calculations and network configuration.

Practical Subnetting Example

Imagine you have the network 192.168.1.0/24 and you need to create 4 subnets.
This network has 8 host bits (/24 means 24 network bits, so 32 - 24 = 8 host bits).

  1. Borrow 2 bits for subnetting (2^2 = 4 subnets).
  2. The new subnet mask becomes /26 (24 original network bits + 2 borrowed bits).
  3. This gives you four subnets: 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, and 192.168.1.192/26.

Each subnet now has 6 bits left for hosts (2^6 - 2 = 62 usable addresses per subnet).