---
title: IP Address
description: What an IP address is and why the internet needs one.
tags: [system-design, fundamentals, networking, ip]
---

## What is an IP address?

* An IP address identifies one device on the internet or on a local network.
* IP stands for Internet Protocol, the rules for how data is formatted when it
  travels over a network.
* The address says where the device sits on the network, so other devices can
  send data to it.
* Without it, nothing on the network could tell one machine from another.

## Versions

### IPv4

The original version. It uses a 32-bit number written in dot-decimal notation,
which gives about 4.3 billion addresses. That was plenty at first, but the
internet outgrew it.

```
102.22.192.181
```

### IPv6

The newer version, introduced in 1998. Rollout started in the mid-2000s and is
still going.

It uses a 128-bit value written in hexadecimal, which gives about 3.4 x 10^38
addresses. Enough for a long time.

```
2001:0db8:85a3:0000:0000:8a2e:0370:7334
```

A bit holds `0` or `1`, so one bit has 2 possible values.

Hexadecimal uses `0 1 2 3 4 5 6 7 8 9 a b c d e f`, so one hex digit has 16
possible values. Since `2^4 = 16`, one hex digit is exactly 4 bits.

An IPv6 address is 32 hex digits. Drop the colons from the example above and you
can count them:

```
20010db885a3000000008a2e03707334
```

So `32 hex digits x 4 bits = 128 bits`.

Each bit doubles the number of possible addresses, and there are 128 of them.

```
2^128 = 16^32 = 340,282,366,920,938,463,463,374,607,431,768,211,456
      = about 3.4 x 10^38
```

The relationship in one place:

```text
1 bit         -> 2 values
4 bits        -> 16 values
1 hex digit   -> 16 values
32 hex digits -> 128 bits
IPv6          -> 2^128 addresses
```

Hex is a shorter way to write the same bits.

## Types

Two separate things get called types here. Public and private is about how far
the address reaches. Static and dynamic is about whether it changes. Every
address has one of each, so your home connection is usually public and dynamic.

### Public

One address for your whole network. Every device behind it goes out to the
internet with that same address.

Example: the address your ISP gives your router.

### Private

An address for a single device inside your network, like a laptop, tablet, or
phone. It is unique inside that network only.

Example: the addresses your home router hands out to your devices.

### Static

A static address never changes. It is picked once and saved in the network
settings, instead of being handed out fresh each time. ISPs charge extra for
one, since that address stays reserved for you.

Example: hosting a server, or reaching a machine remotely.

### Dynamic

A dynamic address changes over time. A DHCP server assigns it, short for Dynamic
Host Configuration Protocol. This is the most common type because it is cheaper
to run and lets a network reuse addresses as devices come and go.

Example: home internet and personal devices.

## How public and private fit together

Your ISP gives the router one public IP. The router gives every device in the
house a private IP.

Private IPs cannot be reached from the internet. Traffic always goes out through
the router, so a site like Google only ever sees the public IP.

### NAT

NAT stands for Network Address Translation. The router swaps addresses on the
way out and back, and uses port numbers to tell devices apart.

When the laptop and the phone both open Google, the router notes each one:

```text
Public                Private
49.36.100.25:40001 -> 192.168.1.10:5000   (laptop)
49.36.100.25:40002 -> 192.168.1.11:5001   (phone)
```

Google replies to `49.36.100.25:40001`. The router looks up port 40001 in that
table and sends the reply to the laptop.

The lookup is public IP plus port, not the IP alone. That is how many devices
share one public IP.