How to find the first and last address in a block of IP addresses and understand subnetting 🌐
Published: 2023-05-30 18:50:29



In this article, I will explain how to find the first and last address in a block of IP addresses given a network address and a subnet mask. I will also explain what is a subnet, why it is useful, and what is the difference between a default mask and a subnet mask. 🚀

## What is an IP address and a subnet mask? 🤔

An IP address is a unique identifier for a device on a network. It consists of four numbers separated by dots, each ranging from 0 to 255. For example: 205.16.37.39.

A subnet mask is a way of dividing an IP address into two parts: a network part and a host part. The network part identifies the network that the device belongs to, and the host part identifies the device within that network. A subnet mask consists of four numbers separated by dots, each ranging from 0 to 255. For example: 255.255.255.240.

The subnet mask can also be written in a shorthand notation using a slash followed by a number, indicating how many bits are used for the network part. For example: /28.

To find the network part of an IP address, we can use a logical operation called AND between the IP address and the subnet mask. For example:

```
IP address:    205.16.37.39
Subnet mask:   255.255.255.240
Network part:  205.16.37.32
```

## How to find the first and last address in a block of IP addresses? 🧐

A block of IP addresses is a range of consecutive IP addresses that belong to the same network. The first address in the block is the network address, which is obtained by applying the subnet mask to any IP address in the block. The last address in the block is the broadcast address, which is obtained by changing all the bits in the host part to 1.

For example, given the IP address 205.16.37.39/28, we can find the first and last address in the block as follows:

```
IP address:    205.16.37.39
Subnet mask:   255.255.255.240
Network part:  205.16.37.32
Host part:     00000111

First address: 205.16.37.32 (network address)
Last address:  205.16.37.47 (broadcast address)
```

The first and last addresses in the block are usually reserved for special purposes and cannot be assigned to devices.

## What is a subnet and why is it useful? 🙌

A subnet is a smaller network that is created by dividing a larger network into smaller parts using a subnet mask. A subnet can have its own network address, broadcast address, and range of host addresses.

Subnetting is useful for several reasons, such as:

- It allows us to create more networks and hosts within a limited range of IP addresses.
- It improves the performance and security of networks by reducing traffic congestion and isolating faults.
- It simplifies the management and administration of networks by grouping devices logically and hierarchically.

## What is the difference between a default mask and a subnet mask? 🤷‍♂️

A default mask is a subnet mask that corresponds to the original class of an IP address, before any subnetting is done. A default mask has all the bits in the network part set to 1 and all the bits in the host part set to 0.

For example:

- Class A: Default mask - 255.0.0.0
- Class B: Default mask - 255.255.0.0
- Class C: Default mask - 255.255.255.0

A subnet mask is a subnet mask that corresponds to the actual subnetwork that an IP address belongs to, after subnetting is done. A subnet mask can have any number of bits in the network part set to 1 and any number of bits in the host part set to 0.

For example:

- Class A: Subnet mask - 255.240.0.0
- Class B: Subnet mask - 255.255.240.0
- Class C: Subnet mask - 255.255.255.240



| Term | Definition |
| --- | --- |
| IP address | A unique identifier for a device on a network |
| Subnet mask | Divides an IP address into two parts: a network part and a host part |
| Block of IP addresses | A range of consecutive IP addresses that belong to the same network |
| Network address | The first address in a block of IP addresses |
| Broadcast address | The last address in a block of IP addresses |
| Subnet | A smaller network created by dividing a larger network into smaller parts using a subnet mask |
| Default mask | Corresponds to the original class of an IP address before any subnetting is done |
| Subnet mask | Corresponds to the actual subnetwork that an IP address belongs to after subnetting is done |


## Conclusion 🎉

In this article, I have explained how to find the first and last address in a block of IP addresses given a network address and a subnet mask. I have also explained what is a subnet, why it is useful, and what is the difference between a default mask and a subnet mask.

I hope you have learned something new and useful from this article 😊

If you have any questions or feedback, please feel free to leave a comment below 👇 Thank you for reading! 👋