Anatomy of a Domain: Subdomain, SLD, and TLD

## Anatomy of a Domain: Subdomain, SLD, and TLD A domain name looks simple on the surface — `www.example.com` seems like just a string of words separated by dots. But each segment has a specific name, a specific role in the DNS（ドメインネームシステム） hierarchy, and different rules about who controls it and how it can be changed. Breaking a domain into its components helps you understand DNS configuration, troubleshoot problems, set up subdomains correctly, and communicate clearly with developers and hosting providers. ## The Full Picture: A Fully Qualified Domain Name The technical term for a complete, unambiguous domain address is an FQDN（完全修飾ドメイン名） — Fully Qualified Domain Name. A FQDN（完全修飾ドメイン名） specifies the exact location of a resource within the entire DNS hierarchy, with no room for ambiguity. In technical notation, an FQDN（完全修飾ドメイン名） ends with a trailing dot representing the DNS root: ``` www.example.com. ``` That trailing dot is almost never shown in browsers or everyday use, but it exists conceptually at the top of the hierarchy. Let us now work through each component from right to left. ## Layer 1: The DNS Root (.) At the very top of the DNS hierarchy sits the **root** — represented by a single dot (`.`). The root does not appear in domain names as you type them, but it is always there implicitly. The root is managed by ICANN and operated by 13 clusters of root name servers distributed worldwide. These root servers are the starting point for all DNS（ドメインネームシステム） lookups. They do not know the answer to any specific domain query — they only know where to send the query next: the appropriate TLD name servers. ## Layer 2: The Top-Level Domain (TLD) Reading from right to left, the first component you encounter is the TLD（トップレベルドメイン） — the Top-Level Domain. This is the segment after the final dot in a domain name: - `example.**com**` → TLD is `.com` - `wikipedia.**org**` → TLD is `.org` - `bbc.co.**uk**` → TLD is `.uk` - `myblog.**photography**` → TLD is `.photography` The TLD（トップレベルドメイン） is managed by a **registry** — an organization delegated authority by ICANN. For `.com`, that is Verisign. For `.uk`, that is Nominet. The TLD defines the category or geographic scope of the domain: - gTLD（汎用トップレベルドメイン） (generic): `.com`, `.org`, `.net`, `.app`, `.shop` - ccTLD（国別コードトップレベルドメイン） (country code): `.us`, `.uk`, `.de`, `.jp` - sTLD（スポンサー付きトップレベルドメイン） (sponsored): `.edu`, `.gov`, `.museum` You choose the TLD when you register. You cannot change a TLD after registration without registering a new domain. ## Layer 3: The Second-Level Domain (SLD) The SLD（セカンドレベルドメイン） — Second-Level Domain — is the segment immediately to the left of the TLD. This is the part you choose and register: - `**example**.com` → SLD is `example` - `**wikipedia**.org` → SLD is `wikipedia` - `**bbc**.co.uk` → SLD is `bbc` (with `.co` being a "pseudo-TLD" in the UK namespace) The SLD（セカンドレベルドメイン） is where your brand or identity lives. It is the most memorable and most valuable part of a domain name. Once registered, the combination of SLD and TLD (`example.com`) belongs to you exclusively for as long as you renew it. Rules for second-level domains: - May contain letters (a-z), digits (0-9), and hyphens (-) - Cannot begin or end with a hyphen - Cannot contain spaces or special characters (with the exception of internationalized domain names using Punycode encoding) - Maximum length: 63 characters - Minimum length: 1 character (though single-character domains are rare) ## Layer 4: The Subdomain A **サブドメイン** is any segment to the left of the SLD（セカンドレベルドメイン）. Subdomains are created and managed by the domain owner in their DNS settings — you do not register them separately, and they do not cost extra. Common subdomains: | Subdomain | Full Domain | Typical Use | |-----------|-------------|------------| | `www` | `www.example.com` | Main website (traditional) | | `blog` | `blog.example.com` | Blog section | | `shop` | `shop.example.com` | E-commerce store | | `app` | `app.example.com` | Web application | | `api` | `api.example.com` | Developer API endpoint | | `mail` | `mail.example.com` | Webmail interface | | `staging` | `staging.example.com` | Testing environment | | `docs` | `docs.example.com` | Documentation site | | `m` | `m.example.com` | Mobile site (legacy) | You can create any subdomain you like for your domain. Each subdomain needs a corresponding DNS record (usually an Aレコード or CNAMEレコード) pointing to the appropriate server. ### Subdomains vs Subdirectories A subdomain (`blog.example.com`) and a subdirectory (`example.com/blog`) are different ways to organize content. From a visitor's perspective, both work. From an SEO perspective, subdirectories are generally preferred for keeping link equity consolidated, while subdomains are treated more like separate sites by search engines. However, the technical differences matter far less than the quality of your content. ### Multi-Level Subdomains You can chain multiple subdomains: `mail.london.example.com`. The DNS system can handle many levels, though practically speaking, more than two levels is uncommon. ## Layer 5: The Second-Level Domain in ccTLDs Some country code TLDs use a two-segment structure with a "pseudo-TLD" layer: - `bbc.co.uk` — `bbc` is the brand, `.co` is the type (commercial), `.uk` is the ccTLD - `abc.gov.au` — `abc` is the brand, `.gov` is the type (government), `.au` is the ccTLD - `mit.edu.in` — Similar pattern for India In these cases: - `.uk` (or `.au`, `.in`) is the TLD（トップレベルドメイン） - `.co.uk` (or `.gov.au`, `.edu.in`) functions as the effective registration namespace - `bbc` (or `abc`, `mit`) is the brand segment These are technically called **Second-Level Domains** within the registry's namespace, but the branding convention treats them as a combined "effective TLD." ## Putting It All Together ``` ┌─────────────────────────────────────────────┐ │ www.example.com │ │ │ │ │ │ │ │ │ └─── TLD (.com) │ │ │ └─────────── SLD (example) │ │ └─────────────── Subdomain (www) │ └─────────────────────────────────────────────┘ ``` ``` ┌─────────────────────────────────────────────────────────────┐ │ docs.api.example.co.uk │ │ │ │ │ │ │ │ │ │ │ │ │ └─── TLD (.uk) │ │ │ │ │ └────── Pseudo-TLD (.co) │ │ │ │ └────────────── SLD (example) │ │ │ └────────────────── Subdomain (api) │ │ └─────────────────────── Sub-subdomain (docs) │ └─────────────────────────────────────────────────────────────┘ ``` ## Who Controls Each Part? | Component | Controlled By | How to Change | |-----------|--------------|---------------| | Root (`.`) | ICANN | Not applicable | | TLD（トップレベルドメイン） | Registry (e.g., Verisign for `.com`) | Cannot change TLD — must register new domain | | SLD（セカンドレベルドメイン） | Registrant (you) | Cannot change SLD — must register new domain | | サブドメイン | Domain owner (you) | Add/edit DNS records in your registrar or DNS provider | | DNS records | Domain owner (you) | Edit in your DNS control panel | ## Practical Implications **When setting up a new subdomain:** Log into your DNS provider (which may be your ドメインレジストラー or a separate service like Cloudflare) and add an Aレコード or CNAMEレコード for the new subdomain. **When configuring www vs non-www:** Both `www.example.com` and `example.com` (the "apex" or "naked" domain) need DNS records. Typically, `www` uses a CNAMEレコード pointing to the apex, and the apex uses an Aレコード pointing to your server's IP. Many DNS providers support "CNAME flattening" or "ALIAS records" to handle the apex. **When diagnosing DNS problems:** Knowing which layer is misconfigured saves time. If `example.com` works but `www.example.com` does not, the subdomain DNS record is missing. If neither works, the issue is at the SLD level or higher. ## Key Takeaways - A FQDN（完全修飾ドメイン名） like `www.example.com` breaks into: root (`.`), TLD（トップレベルドメイン） (`.com`), SLD（セカンドレベルドメイン） (`example`), and サブドメイン (`www`). - The TLD（トップレベルドメイン） is managed by a registry; you choose it at registration and cannot change it. - The SLD（セカンドレベルドメイン） is your brand name; you register it and cannot change it without registering a new domain. - Subdomains are created freely by the domain owner using DNS records — no extra registration needed. - Understanding domain anatomy helps with DNS configuration, troubleshooting, and SEO decisions. Next, read How DNS Works: The Internet's Phone Book to understand how the DNS uses these components to route traffic, or intro-dns-records to learn how to configure the DNS records that point each part of your domain to the right server.