URL

The WHATWG URL Standard defines URL parsing and serialization. When building tracking URLs, use a URL parser to handle paths and query parameters rather than concatenating unescaped strings.

What Is a URL?

A URL — short for Uniform Resource Locator — is the unique address of a resource on the internet. When you type one into a browser’s address bar or click a hyperlink, the browser uses that address to find and fetch the resource: a web page, an image, a video, a downloadable file, or any other piece of content stored on a server.

The key word is uniform. Every URL follows the same syntax rules, no matter what kind of resource it points to or where in the world that resource lives. That uniformity is what lets any browser on any device locate the same resource using the same string of text.

What a URL Is Not

Confusion between related terms is common. Here is how to tell them apart:

• URL vs. domain name: A domain name (like example.com) is one part of a URL — the human-readable label that identifies a website. A URL is the full address that includes the protocol, domain, path, and everything else needed to reach a specific resource. example.com is a domain; https://example.com/about is a URL.
• URL vs. website: A website is the collection of pages, images, and files hosted under a domain. A URL is the address of one specific piece of that collection. Think of a website as a building and a URL as the street address of a particular room.
• URL vs. IP address: An IP address (like 192.0.2.1) is a numeric label that identifies a device on a network. A URL uses a domain name that the DNS translates into an IP address behind the scenes. You can sometimes use an IP address directly in place of a domain, but URLs exist partly so humans do not have to memorize strings of numbers.
• URL vs. link (hyperlink): A link is the clickable text or element on a page. The URL is the destination address embedded inside that link. You click a link; the browser navigates to the URL.

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How Is a URL Structured?

Every URL is built from a set of components, some mandatory and some optional. Reading a URL from left to right, each part adds more precision about what resource to fetch and how to fetch it.

Take this example URL:

https://www.example.com:443/blog/what-is-a-url?utm_source=google&utm_medium=organic#structure

Scheme (Protocol)

The scheme — https in the example — tells the browser which protocol to use when connecting to the server. It is always followed by a colon and two forward slashes (://).

Modern browsers mark HTTP pages as “Not Secure.” For any site handling user data, logins, or payments, HTTPS is the minimum standard.

Authority: Domain Name and Port

The authority section identifies where the resource lives. It includes:

• Domain name (www.example.com): The human-readable name of the server. It consists of a subdomain (www), a second-level domain (example), and a top-level domain (.com). The Domain Name System (DNS) translates this into an IP address.
• Port (:443): An optional number that specifies which network port to use. HTTP defaults to port 80, HTTPS to 443. Most URLs omit the port because browsers assume the default.

Path to Resource

The path (/blog/what-is-a-url) tells the server which specific file or resource to return. It mirrors a file-system directory structure, though modern web servers often rewrite paths dynamically rather than serving literal files from folders. A URL with only a domain and no path (like https://example.com) typically returns the site’s homepage — the server fills in a default path such as /index.html.

Parameters and Query Strings

Everything after a question mark (?) is the query string. It sends additional data to the server in key-value pairs separated by ampersands:

?utm_source=google&utm_medium=organic

Fragment (Anchor)

The fragment (#structure) points to a specific section within the loaded page. It is never sent to the server — the browser handles it entirely on the client side, scrolling to the element whose id attribute matches the fragment value. Fragments are useful for table-of-contents navigation and for linking directly to a subheading.

URL Components at a Glance

How Does a URL Work?

Typing a URL and pressing Enter feels instant, but several steps happen between the keystroke and the fully loaded page.

DNS: The Internet’s Phonebook

The browser cannot connect to a domain name directly — it needs an IP address. The Domain Name System (DNS) is the distributed directory that maps domain names to IP addresses. When you enter a URL, the browser checks its local cache, then queries a DNS resolver, which may contact root servers, TLD nameservers, and authoritative nameservers until it finds the matching IP address. This process normally takes milliseconds.

Browser Request Flow

1. URL parsing: The browser splits the URL into scheme, authority, path, and other components.
2. DNS lookup: The domain name resolves to an IP address.
3. Connection establishment: The browser opens a TCP connection to the server (and performs a TLS handshake if the scheme is HTTPS).
4. HTTP request: The browser sends a request — usually a GET — that includes the path, query string, and headers.
5. Server response: The server processes the request and returns a status code (200 OK, 301 redirect, 404 not found, etc.) along with the resource content.
6. Rendering: The browser parses the HTML, CSS, and JavaScript, fetches any additional resources (images, fonts, scripts) referenced by the page, and renders the final output.

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How to Use a URL

Typing a URL in the Address Bar

The most direct method: click the address bar at the top of your browser, type or paste the full URL, and press Enter. Modern browsers auto-complete URLs from your history and bookmarks, and most no longer require you to type http:// or https:// — they add the scheme automatically.

Clicking Hyperlinks

Any underlined or highlighted text on a web page, button, or image can contain an embedded URL. Hover over a link (without clicking) to preview the destination URL in the bottom-left corner of most browsers — a simple safety check before you follow it.

Finding the URL of the Page You Are On

The URL of the current page is always visible in the address bar at the top of the browser window. On mobile devices, tap the address bar to see the full URL. You can copy it by selecting the entire string and pressing Ctrl+C (Windows) or Cmd+C (Mac).

Opening URLs on Different Devices

The process is the same across desktops, laptops, tablets, and phones: type or paste the URL into the browser’s address bar. On mobile, the address bar may be at the top (Safari, Chrome) or bottom (some Android browsers). Apps can also open URLs — tapping a link in an email, messaging app, or social media post hands the URL to the device’s default browser.

URLs in Affiliate Marketing and Software

For advertisers, publishers, and affiliate program managers, a URL is not just an address — it is the backbone of tracking, attribution, and revenue.

How Affiliate Links Use URLs

An affiliate link is a URL that carries a unique identifier for the affiliate who shared it. When a visitor clicks that link, the tracking parameters in the URL tell the affiliate platform which affiliate sent the traffic. If the visitor completes a purchase or sign-up, the platform attributes the conversion to that affiliate and calculates the commission.

A typical affiliate URL looks like this:

https://www.merchant.com/product?aff_id=12345&campaign=summer_sale

URL Tracking for Advertisers

Advertisers use URLs with UTM parameters to measure campaign performance across channels. A well-tagged URL answers questions like: Which email subject line generated more clicks? Which social platform delivers the highest conversion rate? Which paid keyword is profitable?

A UTM-tagged URL might look like:

https://www.store.com/landing-page?utm_source=newsletter&utm_medium=email&utm_campaign=spring_launch

What Publishers Need to Know

Affiliates (publishers) receive unique links from each merchant or affiliate network they work with. Best practices include:

• Use clean, readable URLs when possible. Some affiliate platforms offer link cloaking — redirecting through a branded domain — which can improve click-through rates by hiding long, parameter-heavy URLs.
• Test every link before publishing. A broken affiliate link earns nothing.

For Affiliate Program Managers

Affiliate software platforms automate link generation, tracking, and reporting. Key capabilities include:

• Automated link generation: Each affiliate gets a unique URL without manual setup.
• Click and conversion tracking: The platform records every click, ties it to a cookie or fingerprint, and attributes conversions within a defined lookback window.
• Fraud detection: Unusual patterns — such as a single IP generating hundreds of clicks — trigger alerts.
• Cookie tracking: When a user clicks an affiliate link, the platform drops a cookie. If the user returns and purchases later (within the cookie window), the affiliate still gets credit.

URL Safety and Best Practices

How to Check If a URL Is Safe

Not every URL is harmless. Phishing attacks, malware distribution, and scam sites often start with a deceptive link. Before clicking:

1. Check the scheme. Prefer HTTPS. A padlock icon in the address bar means the connection is encrypted, though it does not guarantee the site is legitimate.
2. Inspect the domain carefully. Attackers register look-alike domains: paypaI.com (with a capital I instead of an l) or amaz0n.com. Read the domain from right to left to spot substitutions.
3. Hover to preview. On desktop, hover over any link to see the real destination URL in the status bar before clicking.
4. Use a URL scanner. Services like Google Safe Browsing and VirusTotal let you check a URL against databases of known malicious sites.
5. Be wary of shortened URLs. Services like Bitly and TinyURL hide the destination. Use a link-expander tool to reveal the full URL before visiting.

URL Encoding and Valid Characters

URLs can only contain characters from the ASCII character set. Spaces, non-Latin characters, and certain symbols (like # or % outside their defined roles) must be percent-encoded. For example, a space becomes %20, and the € symbol becomes %E2%82%AC.

This is why you sometimes see URLs full of % codes — the browser decodes them for display, but the raw URL must be ASCII-clean. When creating URLs programmatically, always encode dynamic values to avoid broken links or security vulnerabilities.

SEO Best Practices for URLs

Search engines use URLs as a ranking signal, and well-structured URLs improve click-through rates from search results:

• Use readable words, not ID numbers. example.com/what-is-a-url is better than example.com/?p=472.
• Include the primary keyword in the URL path, but do not stuff.
• Keep it short. Descriptive but concise URLs perform better in search results.
• Use hyphens to separate words, not underscores. Google treats hyphens as word separators.
• Maintain consistent structure across the site. A predictable URL hierarchy helps both users and crawlers.
• Use lowercase. Servers may treat Page and page as different URLs, splitting ranking signals.
• Implement 301 redirects when a URL changes. This preserves link equity and avoids 404 errors.