Introduction

This project serves a toy project to learn how subnetting works and how address ranges can be split into smaller subnets. The implemented code solves the following question:

1. Given an IP-Address and the number of used bits for the subnet mask, how many subnets of a certain size can I create? Example:

   Given the subnet 141.67.128.0/21, how many /23-subnets can I create? 
   

2. The code will also calculate the first and last IP-Address in a given subnet as well as the total number of available IP-Addresses.

###Requirements: TODO

What is subnetting?

A subnet is a small network within a larger network. Subnets are used to make network traffic more efficient and safer. In big IT networks there are a lot of possible routes. To limit the number of routers the network traffic has to pass subnets provide a great tool to limit the number of possible routes within a network by "cutting" out a small portion of the available IP-Addresses and put them into a smaller network - the so-called subnet. This subnet is then a part of the bigger network range but traffic does not leave the subnet, if it is directed at IP-Addresses from the same subnet.

IP Address

Every device that connects to a network (i.e. the internet) is assigned an unique IP (Internet Protocol) address. This IP serves as an identifier that is used every time a packet s sent ot the device. Every IP address consists of 4 so-called octets. Every octet consists of 8 Bit. For example: The first octet of the IP address 10.28.65.20 would be 10 = 00001010. The whole IP address in binary would look like this=

10.28.65.20 = 00001010.00011100.01000001.00010100

Subnetmasks

A subnet mask is similar to an IP address but is only used within a network. Both elements are necessary to route a package to the intended destination. The subnet mask serves as an identifier for a network range in which a certain IP address is located. Or to be ver precise: A subnet mask is the representation of the network portion of an address.

Classful Networks

Every IP address can be divided into a network and host part. This does look like the following: Originally, IP address were designed to be part of a network class ranging from Class A to Class E. Commonly used are the classes A-C. In those classes IP addresses were split after 8 bit (Class A), 16 bit (Class B) and 24 bit (Class C). This leaves 24, 16 or 8 bits for the host identifier. This is also shown in Figure (1)

Classless Networks

Today, most networks are classes to allow for a more flexible sizing of networks. In 1993, Classless Inter-Domain Routing (CIDR) was introduced. Since then, network ranges can be of every size (with a few exceptions). An IP address with CIDR notation allows for a concise notation of an IP address together with its subnet mask. A former class C network would be written as 192.168.2.1/24 where the 24 identify the number of network bits ( the
so-called network prefix).

Why use subnetting?

Subnetting has, among others, the following advantages:

• Reduce the number of unused IP addresses in a subnet by tailoring the subnet size to the actual number of IP addresses needed
• Subnetting improves the network performance since in smaller networks there will be less traffic.

How the calculations are done

Number of usable IP addresses

The number of usuable Ip addresses within a subnet is calculated as follows: num_of_usuable_ips = 2^host_bits - 2

We have to subtract two addresses for network and the broadcast address.

Number of Subnets in an address block

To solve question 1 (Given the subnet 141.67.128.0/21, how many /23-subnets can I create?) the following formula can be used num_of_subnets = 2^(required_subnet_size - reference_block_size) Source: https://www.ittsystems.com/introduction-to-subnetting/

Calculate the list of subnets

After calculating how many subnets of a certain size are contained in an address block, it is possible to count the exact address range for each possible subnet. This can be done in the following steps:

1. Calculate in which octet the subnets exists.
   • First octet: Network prefix [/1, /8]
   • Second octet: Network prefix [/9, /16]
   • Third octet: Network prefix [/17, /24]
   • Fourth octet: Network prefix [/25, /32]
2. Take the biggest network prefix number. E.g.: /16 for a subnet within the second octet.
3. Calculate the block size of the subnet. Remember, the block size represents the number of possible IP addresses within the subnet in question. Formula (equal to the total number of IP addresses): block_size = 2^(reference_block_size - required_subnet_size).
4. Calculate the address ranges by starting with the first provided network and increment by the block size.

#####Example: Given the network 174.53.4.0/24, how many /27 networks exists and what are their address ranges?

0. (Calculate the number of subnets: num_of_subnets = 2^(27 - 24) = 2^3 = 8)
1. The subnet exists within the 4th octet
2. The highest network prefix is 32
3. Block size: block_size = 2^(32 - 27) = 2^5 = 32

Therefore, the possible subnets would be:

• 174.53.4.0/27
• 174.53.4.32/27
• 174.53.4.64/27
• 174.53.4.96/27
• 174.53.4.128/27
• 174.53.4.160/27
• 174.53.4.192/27
• 174.53.4.224/27

Note, that this list only contains the address ranges for the subnets. The usable IP addresses range from first_usable_adr = start_ip +1 (since start_ip == network address) to last_usable_adr = last_ip - 1 = network_adr_next_subnet -2

Source for example: https://www.ittsystems.com/introduction-to-subnetting/

Variable Length Subnet Masks (VLSM)

TODO

###How to run: The following prerequisites are necessary to use the project yourself. I assume that you are using a linux or macOS system:

0. Install virtualenv if not already installed

python3 -m pip install --user virtualenv

1. Create a virtual python environment

python3 -m venv <envname>

2. Activate the environment in your shell

source <envname>/bin/activate

3. Install the requirements

pip install -r requirements.txt

4. In the projects root directory, run

python main.py

Input

TODO

Output

TODO

###Sources:

• https://www.ittsystems.com/introduction-to-subnetting/
• http://www.subnetmask.info/
• https://en.wikipedia.org/wiki/Subnetwork
• https://www.freecodecamp.org/news/subnet-cheat-sheet-24-subnet-mask-30-26-27-29-and-other-ip-address-cidr-network-references/
• https://subnettingpractice.com/vlsm.html
• https://www.baeldung.com/cs/get-ip-range-from-subnet-mask
• https://erikberg.com/notes/networks.html
• https://ljvmiranda921.github.io/notebook/2018/06/21/precommits-using-black-and-flake8/