Domain

Key points

• A domain is a human-readable address built from a hierarchy: "top-level domain," "second-level domain," and "subdomain."
• A browser converts the "hostname" in a URL into an "IP address" via "DNS," then connects to the target server.
• This conversion process is called "address resolution" (or "name resolution").

Domain structure

A domain is a human-readable name that indicates the location of a server on the internet. Actually identifying a server requires a string of numbers called an "IP address," but numbers alone are hard to remember, so the domain — a string of characters — is used instead.

A domain is made up of several parts joined by periods, and it forms a hierarchy where, reading from right to left, the scope narrows step by step.

We'll look at what each of these three parts means, one by one, over the next few slides.

Teacher Pochi's hintIt's like a postal address written from largest to smallest — country, state, city, street. A domain works the same way, just in reverse order: the rightmost part is the broadest category (country or organization type), and it gets more specific as you move left.

Top-level domain

The top-level domain (TLD) is the rightmost part of a domain — things like ".com" or ".jp" — and it represents a broad category, such as a country or an organization type. There are two main kinds.

Teacher Pochi's hintIt's like the "Inc." or "Ltd." attached to a company's legal name — a label for what kind of entity it is. ".com" broadly signals a commercial business, ".org" a nonprofit, and so on.

Second-level domain

The second-level domain (SLD) sits just to the left of the top-level domain and corresponds to an organization's or service's name — the "example" in example.co.jp. It can be freely registered on a first-come, first-served basis, as long as it isn't already taken.

Teacher Pochi's hintIt's like the company name on a storefront sign. Within a given entity type ("Inc." — the top-level domain), the second-level domain is what tells you exactly which company it is.

Subdomain

A subdomain is a mechanism for splitting a single domain up by purpose. Even within the same organization's domain, changing the subdomain lets you operate separate services or departments.

Teacher Pochi's hintThis is a lot like a large building (example.com) with multiple tenants inside (each subdomain). While sharing the same building address, they're divided up much like room numbers — "1st floor is a shop," "2nd floor is an office."

Address resolution

Before a browser can actually connect to a server, it has to convert the domain — a human-facing string — into the numeric address computers use: the "IP address." This conversion process is called "address resolution" (or "name resolution").

We'll look at the role each of these — URL, hostname, DNS, and IP address — plays in this process, one by one, over the next few slides.

Teacher Pochi's hintIt's like typing a destination address into a GPS: behind the scenes, it gets converted into latitude and longitude before the directions start. We type in something easy to understand (the domain), but what's actually used is a set of numeric coordinates (the IP address).

URL

A URL is the full string that indicates the location of something on the Web. It includes the domain we've been looking at so far.

The "https://" part is the communication method (the scheme), the middle part is the domain (hostname) we've been covering, and the rest — the "path" — indicates a location within the server.

Teacher Pochi's hintThink of the whole URL as "the entire address on an envelope." The domain part within it is "the building's street address," and the path is like the "room number" inside that building.

Hostname

A hostname is the name given to a single computer (a "host") connected to a network. For web servers, it's traditionally prefixed to the domain as "www," and "hostname + domain" together form what's called the "FQDN" (fully qualified domain name).

It looks a lot like a subdomain, but there's a technical distinction: a subdomain is about "dividing things up by purpose," while a hostname refers to "the name of an individual machine."

Teacher Pochi's hintIt's like the nameplate on each individual room within a building (the domain). The building's address alone doesn't tell you whose room is whose.

DNS

DNS (Domain Name System) is the mechanism that converts a domain (hostname) into its actual IP address. When a browser accesses a domain, a query to DNS happens behind the scenes.

We cover DNS in more depth in the "DNS" topic under Infrastructure & Network Fundamentals.

Teacher Pochi's hintDNS is like a "phone book." If you know someone's name (the domain) but not their phone number (the IP address), you look it up in the phone book before dialing. DNS plays exactly the same role.

IP address

An IP address is a numeric address assigned to each device connected to the internet. It's written in a form like 192.0.2.1, and it's this IP address — not the domain — that's actually used in communication.

The format and types of IP addresses (such as IPv4 versus IPv6) are covered in more depth in the "TCP/IP" topic under Infrastructure & Network Fundamentals.

Teacher Pochi's hintIt's the "phone number itself" you find by looking someone up in the DNS phone book. What you actually dial with isn't a person's name — it's this number.

Summary

A domain is a human-facing address that points to a server, built from a hierarchy of top-level domain, second-level domain, and subdomain. In actual communication, "address resolution" — DNS converting the hostname in a URL into an IP address — is what lets you reach the intended server.

Related topics:

• Request and Response
• Protocol (HTTP)
• DNS
• TCP/IP
• Cookie
• Session
• Cache
• What Are Web Fundamentals?

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