URL Structure and DNS Hierarchy

Every web address is built from distinct parts. Understanding how they fit together — and how DNS resolves them — makes troubleshooting connectivity and naming issues much easier.

1. Anatomy of a URL

Take this example:

http://www.cisco.com/index.html

Each segment has a specific role:

Part	Name	Role
http://	Protocol / scheme	Rules the browser and server use to communicate. Here: HyperText Transfer Protocol. Other examples: https://, ftp://, ssh://.
www	Subdomain / host	Identifies a specific server or service within the organization’s domain. Could also be mail.cisco.com, store.cisco.com, etc.
cisco	Second-Level Domain (SLD)	The main name of the site — the brand or organization that registers and owns the domain.
.com	Top-Level Domain (TLD)	The highest category in the Internet naming hierarchy. Groups domains by purpose (.com commercial, .edu educational, .gov government) or by country (.es, .mx, .uk).
/index.html	Path and file	Once the correct server is reached, this tells it exactly which document or directory to return.

The Fully Qualified Domain Name (FQDN) is the complete hostname: www.cisco.com — including every label from the host down to the root.

2. DNS reads right to left

The Domain Name System (DNS) resolves names by walking the hierarchy from right to left, stopping just before the first / in a URL.

For www.cisco.com:

.com  →  cisco  →  www
 TLD      SLD     host

Think of DNS as a postal system:

1. TLD (.com) — the country or district: which global namespace does this name belong to?
2. SLD (cisco) — the street: which registered organization owns this name?
3. Subdomain (www) — the building number: which specific server or service inside that organization?

Only after DNS returns an IP address for www.cisco.com does the browser open a TCP connection and send an HTTP request for /index.html.

3. The DNS hierarchy

                    .  (root)
                    |
          +---------+---------+
          |         |         |
        .com       .org       .es
          |         |         |
        cisco     wikipedia  google
          |
    +-----+-----+
    |           |
   www         mail

Each level is managed by a different authority:

Level	Example	Managed by
Root (.)	—	Root DNS servers (IANA)
TLD	.com, .org, .es	Registry operators (Verisign, Red.es, etc.)
SLD	cisco.com	The organization that registered the domain
Subdomain	www.cisco.com	The organization itself (via its DNS zone file)

When you register cisco.com, you control all subdomains beneath it: www.cisco.com, api.cisco.com, vpn.cisco.com, and so on.

4. Common TLD categories

Type	Examples	Purpose
Generic (gTLD)	.com, .org, .net, .io	General use; .com is the most common for commercial sites
Sponsored	.edu, .gov, .mil	Restricted to specific communities (education, US government, military)
Country-code (ccTLD)	.es, .mx, .de, .uk	Tied to a specific country or territory

The correct TLD for a commercial company like Cisco is .com.

5. What DNS does not resolve

DNS only maps hostnames to IP addresses. It does not handle:

• The protocol (http:// vs https://) — chosen by the browser or application.
• The path (/index.html) — sent by the browser after the TCP/TLS connection is established.
• The port — defaults to 80 for HTTP and 443 for HTTPS unless specified (https://example.com:8443).

http://www.cisco.com/index.html
|____||_______________||__________|
 proto   DNS resolves     HTTP path
         this part        (not DNS)

6. Quick reference

http://www.cisco.com/index.html
└─┬─┘ └──────┬──────┘ └────┬────┘
protocol   FQDN         resource path
           (DNS scope)
              └─┬─┘└┬┘
               SLD TLD
              └┬┘
            subdomain

1. DNS resolves www.cisco.com → IP address (e.g., 198.18.1.1).
2. Browser connects to that IP on port 80 (HTTP).
3. Browser sends: GET /index.html HTTP/1.1.
4. Server returns the HTML document.

7. Verification

Query DNS from a terminal:

nslookup www.cisco.com

dig www.cisco.com

Trace the full resolution path:

dig +trace www.cisco.com

On Cisco IOS:

Router# ping www.cisco.com

Router# show ip dns view