Glossary · 2 min read

What Is a Domain Name?

Learn what a domain name is, how domains map to IP addresses via DNS, the difference between TLD types, and how subdomains, URLs, and hostnames relate.

What Is a Domain Name?

A domain name is a human-readable address that identifies a location on the internet. Instead of remembering an IP address like 93.184.216.34, you type example.com into your browser, and the Domain Name System (DNS) translates it to the correct IP. Domain names are the addressing system that makes the internet usable.

Domain Name Structure

A domain name is read from right to left, with each level separated by a dot:

blog.example.com
 |      |      |
 |      |      +-- TLD (Top-Level Domain)
 |      +--------- SLD (Second-Level Domain)
 +---------------- Subdomain
  • TLD (Top-Level Domain): The rightmost label (.com, .org, .net, .io). Managed by domain registries.
  • SLD (Second-Level Domain): The label you register (example). Combined with the TLD, this forms the registerable domain (example.com).
  • Subdomain: Any label prepended to the registerable domain (blog, www, mail). You can create unlimited subdomains within your domain.

The full string blog.example.com is a Fully Qualified Domain Name (FQDN) when written with a trailing dot (blog.example.com.), indicating the root of the DNS hierarchy.

How Domains Map to IP Addresses

Domain names are resolved to IP addresses through DNS lookups:

  1. Your device queries a recursive DNS resolver for the A (IPv4) or AAAA (IPv6) record of the domain.
  2. The resolver traces the DNS hierarchy: root servers, TLD servers, then the domain's authoritative name servers.
  3. The authoritative server returns the IP address.
  4. Your device connects to that IP.

For email, the process uses MX records instead of A records, directing mail to the domain's designated mail servers. Verify your domain's DNS configuration with our DNS lookup tool.

Types of Top-Level Domains

gTLDs (Generic Top-Level Domains)

The original gTLDs include .com, .org, .net, .edu, .gov, and .mil. Since ICANN's 2012 expansion, over 1,200 new gTLDs have been introduced, including .dev, .app, .io, .blog, and .shop.

ccTLDs (Country-Code Top-Level Domains)

Two-letter codes assigned to countries and territories per ISO 3166-1: .uk (United Kingdom), .de (Germany), .jp (Japan), .au (Australia). Some ccTLDs like .io, .ai, and .co have been adopted for general commercial use beyond their original country.

Infrastructure TLD

The .arpa TLD is reserved for internet infrastructure purposes, most notably reverse DNS lookups (in-addr.arpa for IPv4, ip6.arpa for IPv6).

Domain vs. URL vs. Hostname

These terms are related but distinct:

  • Domain name: example.com -- the registered name.
  • Hostname: mail.example.com -- a specific host within the domain (includes subdomains).
  • URL (Uniform Resource Locator): https://www.example.com/page?id=1 -- a complete address including protocol, hostname, path, and query parameters.

Domains and Email

Your domain name is the identity behind every email you send. The domain appears in your From address, your authentication records (SPF, DKIM, DMARC), and your MX records. A properly configured domain with clean DNS records is the foundation of email deliverability.

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More glossary terms

DNS Ports Explained

Understand which network ports DNS uses, including port 53 for standard queries, port 443 for DNS over HTTPS, and port 853 for DNS over TLS.

DNS Record Types: A Complete Reference Guide

A technical reference for all major DNS record types including A, AAAA, MX, CNAME, TXT, NS, SOA, PTR, SRV, and CAA. Includes example values and use cases.

How Do IP Addresses Work?

Understand how IP addresses work, including IPv4 vs IPv6 formats, public vs private addresses, static vs dynamic assignment, NAT, and CIDR notation.

SMTP Ports Explained: 25, 465, 587, and 2525

Understand the four SMTP ports, when to use each one, the difference between STARTTLS and implicit TLS, and why ISPs block port 25.

What Is DKIM (DomainKeys Identified Mail)?

Learn how DKIM uses cryptographic signatures to authenticate email, including selector records, public/private key pairs, and the verification process per RFC 6376.

What Is DMARC?

Understand DMARC, how it builds on SPF and DKIM with alignment checks, policy options (none, quarantine, reject), and aggregate vs forensic reporting per RFC 7489.

What Is DNS (Domain Name System)?

Learn how the Domain Name System works, including recursive and iterative queries, root servers, caching, and TTL values. A technical overview referencing RFC 1035.

What Is Secure DNS (DoH and DoT)?

Learn about secure DNS protocols including DNS over HTTPS, DNS over TLS, and DNSSEC. Understand why standard DNS is insecure and how to protect your queries.

What Is a Gateway IP Address?

Learn what a gateway IP address is, how default gateways route traffic between networks, and how to find your gateway address on any operating system.

What Is a Reverse Lookup?

Understand reverse lookups including reverse DNS (PTR records), reverse IP lookups, and reverse email lookups. Learn how each works, common use cases, and limitations.

What Is a Reverse Proxy?

Learn how reverse proxies work, the difference between forward and reverse proxies, and common use cases including load balancing, SSL termination, and caching.

What Is an Email Bounce?

Understand email bounces, the difference between hard and soft bounces, common SMTP error codes, and how bounces affect sender reputation.

What Is an Email Domain Name?

Learn what an email domain is, how the @domain.com part of email addresses works with MX records to route messages, and the difference between free and custom email domains.

What Is an SMTP Server?

Learn how SMTP servers work, the difference between MTA, MSA, and MDA, and how email is delivered hop by hop using the SMTP protocol defined in RFC 5321.

What Is an SPF Record?

Understand SPF records, how they authorize email senders, the TXT record format, and how receivers evaluate SPF results including pass, fail, and softfail.

What Is an SSL Certificate?

Learn how SSL/TLS certificates encrypt internet traffic, how the chain of trust works, the difference between DV, OV, and EV certificates, and why TLS replaced SSL.