Marked in the browser

![nets](https://images.pexels.com/photos/2881229/pexels-photo-2881229.jpeg?auto=compress&cs=tinysrgb&dpr=3&h=750&w=1260)

# Subnets and IPv4 addresses

Just a refresher of what is an IPv4 address and how it is used in subnets.

## IPv4 address

An IP address is an identification number given to a device and this device is connected to a network that uses internet protocol.

In use since the 1980s, IPv4 is the most popular version of IP(Internet Protocol). IPv6, the most recent version, is still far behind in popularity.

> The biggest problem of IPv4 is that it can only represent 4 billion IP addresses. Therefore it is impossible to give a static unique IP to everyone on Earth. In comparison IPv6 can deliver around 340 undecillion, or 340 billion billion billion billion, addresses.

The strength of IPv4 is in its simplicity, which we will go over now.

## Format

An IPv4 address is a number. It is a single binary number of 32 bits (0s and 1s).

The usual way to represent such number is with 4 octets. An octet is a binary number of 8 bits with a range of values from 0 to 255.

Here are some examples of IP addresses:

- 127.0.0.1
- 192.168.0.1
- 24.233.110.10

## Classful network

Just for historical reason, I will mention here that in the earlier times of IPv4, IP addresses were categorized in different classes.

Addresses were classified and subnets were masked based on the first octet value (masks will be explained later):

class|first octet range|default mask
-----|-----------------|------------
A | 1-126 | 255.0.0.0
B | 128-191 | 255.255.0.0
C | 192-223 | 255.255.255.0
D | 224-239 Multicasting
E | 240-255 Experimental
loopback | 127

This convention is still a followed tradition today, except we dont talk about classes anymore. But our home router address is usually setup to 192.168.0.1 with a default mask at 255.255.255.0 and localhost(127.0.0.1) is on the loopback reserved spot.

## CIDR

Starting in 1993, CIDR (Classless Inter Domain Routing), was introduced in hopes of slowing down the exhaustion of IPv4 addresses.

### Variable length subnet masking

One concept introduced with IPv4/CIDR is the variable length subnet masking.

By changing the length of the mask, it becomes possible to create greater networks with more IP addresses in a subnet.

### Notation

Notation is basically same as explained above for IPv4. We have 4 octets separated by dots, but then follows "/" and the amounts of bits to the mask (0-32)

Here are some examples :
- 0.0.0.0/0
- 192.168.0.15/24

## Subnet masking

![mask](https://i.imgflip.com/titme.jpg)

The length in bits of the mask decides how many IPv4 addresses are available on the network.

1. A 24 bits long mask is represented by 24 consecutive 1s(in a 32 bits binary number) or in octets would be :
- 255.255.255.0
- 32 minus 24, that just leaves 8 bits of possible IP addresses(256) on this network.

2. A 16 bits long mask is represented by 16 consecutive 1s, or :
- 255.255.0.0
- 32 minus 16, that leaves 16 bits of possible IP addresses on this network(65536).

We apply a "AND" bitwise operation between the IPv4 address and the mask. That gives us the **network ID**.

It is as if we cut the binary IP address in two parts. The left part is the **network ID**, the right part is the **host ID**.

## Special cases

A few special cases

### The network ID

The first IP address available of a subnet is reserved as the **network ID**.

### The broadcast

The last IP address available of a subnet is reserved as the **broadcast IP address**. The broadcast is used to message all IP addresses of a subnet.

There is also "zero network broadcast" 255.255.255.255 which can be used to message all nodes of this network.

### The loopback

All IP address starting with 127 are part of the **loopback** network. Is not really a network, but just a single computer that talks to itself.

For example, if you try command line:

```
ping 127.0.0.1
```
or
```
ping 127.255.255.254
```

You get response back from yourself.

© Paul Caron, aka ChillPill

IPv4 and subnets