🔀 Converting IPv4 to IPv6

What that ::ffff:192.168.1.1 entry in your logs actually is.

You may have seen an IPv4 address show up in IPv6 form in a system log or network config, like ::ffff:192.168.1.1. Here's what that notation means.

What an IPv4-mapped IPv6 address is

This is a special format (RFC 4291) for representing an IPv4 address within IPv6 address space in dual-stack environments. The first 80 bits are 0, the next 16 bits are all 1 (ffff), and the last 32 bits hold the IPv4 address directly, producing ::ffff:a.b.c.d.

How the conversion works

Just carry the IPv4 address's four octets straight into the last 32 bits. For example, 192.168.1.1 becomes ::ffff:192.168.1.1. Written purely in hex it would be ::ffff:c0a8:0101, but by convention the last 32 bits are kept in the original dotted-decimal form.

Where you'll run into it

This shows up often when analyzing connection logs on a dual-stack server, handling legacy IPv4 clients on an IPv6-only network, or in socket programming when an AF_INET6 socket is configured to also accept IPv4 connections (which then appear in mapped form).

Something to keep in mind

This mapped address isn't a true "native" IPv6 address — it's a compatibility notation, not used for actual internet routing. It's mainly for a local system to handle IPv4 and IPv6 sockets through one unified code path.

More guides

How to Find a Domain's IP Address

Find the real server IP a domain points to

How to Check DNS Propagation (What Is TTL?)

Understand TTL and check propagation after a nameserver change

The Easiest Way to Find Your IP Address

Public vs private IP, and how to check yours

CIDR Notation and How to Calculate a Subnet Mask

Convert CIDR notation like /24 into a subnet mask

How to Check an SSL Certificate's Expiration Date

Check a certificate's expiry without opening a browser

Understanding JWT Structure and How to Decode It

The Header, Payload & Signature — explained

How to Create an Apache .htpasswd File for Basic Auth

Generate an APR1-MD5 hashed .htpasswd file

What Makes a Strong Password (and How to Generate One)

Length, character sets & entropy explained

What Is Base64 Encoding and When to Use It

Why it's used in email attachments & data URIs

Using a JSON Formatter to Read API Responses

Turn minified JSON into something readable

How to Convert a Unix Timestamp to a Date

Turn seconds-since-1970 into a readable date

How to Format Messy SQL Queries

Break a one-line SQL statement into readable clauses

Converting Between HEX, RGB, and HSL Color Codes

The difference between the three formats

camelCase, snake_case & Other Naming Conventions

When to use each naming style

How to Compare Two Text Files (Diff Checker)

Quickly spot differences between two versions

What Is Punycode?

The encoding that makes internationalized domain names (IDN) safe for ASCII-only DNS.

Converting IPv4 to IPv6

What that ::ffff:192.168.1.1 entry in your logs actually is.

Extracting Domains From Text

Pulling just the domains out of mixed text like logs or emails.

Opening Multiple URLs at Once

Stop clicking through your daily site list one at a time.

Verifying an Email Address

What format checks and MX records can confirm — and what they can't.

Inspecting HTTP Response Headers

Cache policy, server details, and compression, all from one set of headers.

Checking Security Headers

HSTS, CSP, X-Frame-Options — and the risk of skipping each one.

Looking Up Domain Info With WHOIS/RDAP

Who registered it, when, and when it expires.

Finding a Rough Location From an IP

How it works, how accurate it really is, and why a VPN changes the answer.

Checking OGP Meta Tags

What to check when a share card on social or chat apps looks wrong.

Checking Heading Structure (h1–h6)

The foundation of SEO and accessibility — a clean heading hierarchy.

Checking TITLE & META Tags

The tags that decide how a page appears in search results.

What Is a Hash Function?

Which hash to use when, and what "can't be reversed" actually means.

How QR Codes Work

Why they still scan when partly covered, and how QR phishing works.

Reading SSL Certificate Fields

What CN, SAN, issuer, and validity period each mean.

Why getting an SSL certificate starts with generating a CSR.

PEM vs DER Certificate Format

Why the same certificate sometimes needs to be re-encoded.

When Cert and Key Don't Match

How the check works, and what usually causes a mismatch.

Using a Self-Signed SSL Certificate

Fine for testing and dev — never for production. Here's why.

HTTP Basic vs Bearer Auth

How the Authorization header value is built, and which scheme fits which case.

Generating Secure Random Tokens

How long should an API key or session secret be, and in what format?

Encrypting Config Values With Jasypt

How the ENC(...) value in application.properties actually works.

What Is JSON String Escaping?

Safely putting text with quotes or newlines into a JSON value.

Formatting vs Minifying JS/CSS

Format while you work, minify when you ship — and why they pull in opposite directions.

How Far Minification Should Go

Stripping whitespace is always safe — renaming variables can break things.

Why HTML Indentation Matters

You can't spot a mistake in a structure you can't see.

When to Reformat XML

Seeing the element nesting in a single dense line of XML.

What Are HTML Entities?

Showing literal <, >, and & characters on the page.

Turning a Spreadsheet Into an HTML Table

From copy-paste straight to HTML — no tags typed by hand.

Extracting Links From HTML

Pulling out every href and src value at once.

The Limits of PHP/MySQL Syntax Checkers

What a bracket/quote check can confirm — and what it can't.

When a MySQL Pairing Check Helps

Catching mistakes in long subqueries and dynamically built queries.

Common HTML5 Validation Errors

Mismatched tags, duplicate ids, missing alt — why each one matters.

When to Use a Visual HTML Editor

Where WYSIWYG wins, and where writing code by hand wins.

Why Markdown Preview Matters

What the raw code hides that the rendered result reveals.

What Is URL Percent-Encoding?

Why a space or non-ASCII character turns into something like %20 or %EC%95%88.

Find & Replace With Regex

For replacements a plain search can't do.

What to Know Before Sorting a List

Why alphabetical and numeric order don't behave the same way.

Extracting Emails From Text

The regex pattern, and the false positives that trip people up.

Why Character Count ≠ Byte Count

Why one Korean character can take up 3 bytes.

What Are Fullwidth and Halfwidth Characters?

Why something that looks like a digit doesn't match in search.

What Is a Unicode Code Point?

The unique number behind every character, and how to read U+XXXX.

How Text Converts to Numbers

Decimal, hex, binary — the same value, written three ways.

UTF-8 Byte Encoding Basics

Why English takes 1 byte and Korean takes 3.

How Text Becomes 0s and 1s

What a bit is, and how text turns into bits.

How ASCII Art Is Made

Drawing large letters out of nothing but characters.