What Is My IP Address? IPv4 vs IPv6 Explained for DevelopersPublished

What Is My IP Address? IPv4 vs IPv6 Explained for Developers
If you've ever debugged a CORS error, set up an IP allowlist, or wondered why req.ip returned something weird in your Express logs, you've run into the same question from a different angle: what actually is an IP address, and which one is "mine"? fastestchecker.com

This post breaks down IPv4 vs IPv6, public vs private IPs, and how to reliably detect a user's IP address in your own code — plus a fast way to check yours right now.fastestchecker.com

TL;DR
IPv4 addresses look like 192.168.1.1 — four numbers, 0-255, separated by dots. There are about 4.3 billion of them, and we've run out.
IPv6 addresses look like 2001:0db8:85a3::8a2e:0370:7334 — a much larger address space designed to replace IPv4.
Your device usually has a private IP (local network) and shares a public IP (internet-facing) with everyone else on your router.
You can check your current public IP instantly with a tool like FastestChecker's IP Checker — useful for confirming what your server or API actually sees.

> IPv4 vs IPv6
: What's the Actual Difference
IPv4
IPv4 has been the backbone of the internet since the 1980s. It's a 32-bit address, which caps the total number of unique addresses at roughly 4.3 billion. Given how many devices are online today, that pool has been effectively exhausted for years — which is why NAT (Network Address Translation) exists: it lets an entire household or office share one public IPv4 address.

Example IPv4: 203.0.113.42

IPv6

IPv6 uses 128-bit addresses, which gives it an address space so large it's effectively unlimited for practical purposes (2^128 addresses). It was designed specifically to solve IPv4 exhaustion, and adoption has been climbing steadily — most major cloud providers and mobile carriers support it by default now.
**
Example IPv6:** 2001:0db8:85a3:0000:0000:8a2e:0370:7334

Quick Comparison

IPv4IPv6Address length32-bit128-bitFormatDotted decimal (192.168.1.1)Hexadecimal, colon-separatedTotal addresses~4.3 billion~340 undecillionNAT required?Usually, yesNo — every device can have a unique public addressAdoptionLegacy, still dominantGrowing, especially on mobile networks

Public IP vs Private IP

This trips up a lot of people building their first networked app:
Private IP — assigned to your device on your local network (home Wi-Fi, office LAN). Ranges like 192.168.x.x, 10.x.x.x, and 172.16.x.x–172.31.x.x are reserved for this and never routed on the public internet.
Public IP the address your router uses to talk to the internet. This is what external servers see when your device makes a request, and it's typically shared across every device on your network via NAT.

If you console.log a request's IP in a local dev environment and see something like ::1 or 127.0.0.1, that's the loopback address — your machine talking to itself.

How to Get a User's IP Address in Your Backend

A common point of confusion: the "IP address" your server sees isn't always the real client IP, especially behind a proxy, load balancer, or CDN (like Cloudflare or an Nginx reverse proxy). Here's how to handle it properly.

Node.js / Express

`javascriptapp.get('/whoami', (req, res) => {
// Direct connection IP
const directIp = req.socket.remoteAddress;

// If behind a proxy/load balancer, check the forwarded header
const forwardedIp = req.headers['x-forwarded-for']?.split(',')[0].trim();

res.json({ ip: forwardedIp || directIp });
});`

X-Forwarded-For can contain a comma-separated list if the request passed through multiple proxies — the first value is generally the original client IP, but treat it as untrusted input unless you control the proxy chain.

*Python / Flask
*

pythonfrom flask import request

@app.route('/whoami')
def whoami():
    ip = request.headers.get('X-Forwarded-For', request.remote_addr)
    return {"ip": ip.split(',')[0].strip()}

A Note on Trust

Never use a client-reported IP for security-critical decisions (like rate limiting or geofencing) without validating it against a trusted proxy layer first. Headers like X-Forwarded-For can be spoofed by the client unless your infrastructure strips and re-sets them at the edge.

Why Developers Actually Need to Check Their IP

A few real scenarios where this matters day to day:

Debugging API allowlists — confirming the exact public IP your server or CI pipeline is making outbound requests from, so you can whitelist it correctly.
Testing geolocation logic — verifying what country/region your current IP resolves to before shipping geo-based features.
VPN/proxy verification — checking that a VPN or proxy is actually masking your IP as expected.
Local network troubleshooting — confirming whether you're getting a private or public address, and whether IPv6 is actually being assigned by your ISP.
For a fast, no-signup way to check this, FastestChecker's IP Checker shows your current public IPv4/IPv6 address, along with basic connection info — handy for a quick sanity check without spinning up a curl command.
If you want to double check via terminal instead:
bashcurl -4 ifconfig.me # Force IPv4
curl -6 ifconfig.me # Force IPv6
FAQ
Does everyone with the same Wi-Fi have the same public IP?
Yes, typically. Devices on the same home or office network share one public IP address via NAT, even though each device has its own private IP internally.
Why do I have both an IPv4 and IPv6 address?
Most modern networks run in "dual-stack" mode, assigning both an IPv4 and IPv6 address so your device can connect either way depending on what the destination server supports.
Can two devices have the same public IP at the same time?
Yes — this is normal for devices behind the same NAT (same home/office network). It's the combination of IP + port that uniquely identifies each connection.
Is my IP address the same as my location?
Roughly — IP-based geolocation is usually accurate to the city or region level, tied to your ISP's registered address blocks, but it's not a precise GPS location.
Auther: Muhammad Asad Arshad
Email: [e-mail]