10  URL Anatomy - Crash Course

A URL (Uniform Resource Locator) is the address system for resources on the internet. Let me break down its components systematically.

10.1 Complete URL Structure

https://user:pass@api.example.com:8443/v1/patients?id=123&format=json#results
└─┬─┘   └───┬───┘ └──────┬───────┘└┬─┘ └──────┬──────┘ └────────┬────────┘ └──┬──┘
scheme  userinfo      host       port      path            query          fragment
  │         │           │          │         │                │                │
  └─────────┴───────────┴──────────┴─────────┴────────────────┴────────────────┘
                            authority                  resource

10.2 Scheme (Protocol)

https://
└─┬─┘
scheme

Purpose: Defines the protocol for accessing the resource

Common schemes:

• http:// - Hypertext Transfer Protocol (unencrypted)
• https:// - HTTP Secure (encrypted with TLS/SSL)
• ftp:// - File Transfer Protocol
• ws:// / wss:// - WebSocket (unencrypted/encrypted)
• file:// - Local file system
• mailto: - Email address
• tel: - Telephone number

10.3 Authority

The authority section contains information about who/where to connect:

user:pass@api.example.com:8443
└───┬───┘ └──────┬───────┘ └┬─┘
userinfo      host        port

10.3.1 Userinfo (Optional)

user:pass@
└─┬┘ └┬┘
user password

Purpose: Credentials for authentication (rarely used in modern web, security risk)

Example: ftp://admin:secret@ftp.example.com

⚠️ Security Note: Embedding credentials in URLs is deprecated and insecure. Use proper authentication methods instead.

10.3.2 Host (Required)

api.example.com
│   │      │
│   │      └── TLD (Top-Level Domain)
│   └────────── Second-Level Domain
└────────────── Subdomain

Can be:

• Domain name: www.example.com
• IPv4 address: http://192.168.1.1
• IPv6 address: http://[2001:0db8:85a3::8a2e:0370:7334]

Domain hierarchy (right to left):

pacs.radiology.ramathibodi.ac.th
 │      │          │        │ │
 │      │          │        │ └─ TLD (top-level)
 │      │          │        └─── Second-level
 │      │          └──────────── Third-level
 │      └─────────────────────── Fourth-level (subdomain)
 └────────────────────────────── Fifth-level (subdomain)

10.3.3 Port (Optional)

:8443
 └┬─┘
 port number

Purpose: Specifies which port to connect to on the host

Default ports (when omitted):

• http:// → port 80
• https:// → port 443
• ftp:// → port 21
• ws:// → port 80
• wss:// → port 443

Example: https://localhost:3000 (development server)

10.4 Path

/v1/patients/123/studies
└┬┘ └──┬───┘ └┬┘ └──┬──┘
 │     │      │     └──── resource
 │     │      └────────── ID parameter
 │     └───────────────── collection
 └─────────────────────── API version

Purpose: Identifies the specific resource on the host

Characteristics:

• Hierarchical structure separated by /
• Case-sensitive on most servers
• Can represent physical file paths or logical routes
• Empty path defaults to /

10.5 Query String

?id=123&format=json&sort=date
 └─┬─┘ └─────┬─────┘ └───┬───┘
   │         │            │
   └─────────┴────────────┴─── key=value pairs

Purpose: Passes parameters to the resource

Structure:

• Starts with ?
• Multiple parameters separated by &
• Format: key=value
• URL-encoded special characters

URL Encoding Examples:

Space       → %20 or +
&           → %26
=           → %3D
?           → %3F
/           → %2F
Thai: ก     → %E0%B8%81

10.6 Fragment (Hash)

#section-results
 └──────┬─────┘
     fragment

Purpose: Points to a specific section within the resource

Key characteristics:

• Starts with #
• Not sent to server (client-side only)
• Used for in-page navigation
• Used in Single Page Applications (SPA) for routing

10.7 Complete Example Analysis

Let’s analyze a PACS (Picture Archiving and Communication System) URL:

https://admin@pacs.hospital.ac.th:8443/viewer/study/1.2.840.113619?series=1#image-3

├─ https://                    → Secure HTTP protocol
├─ admin@                      → Username (password omitted)
├─ pacs.hospital.ac.th         → PACS server hostname
├─ :8443                       → Custom HTTPS port
├─ /viewer/study/1.2.840...    → Path to DICOM study viewer
├─ ?series=1                   → Query: specific series number
└─ #image-3                    → Fragment: scroll to 3rd image

10.8 URL vs URI vs URN

                    URI (Uniform Resource Identifier)
                              │
              ┌───────────────┴───────────────┐
              │                               │
            URL                             URN
    (Uniform Resource Locator)    (Uniform Resource Name)
    "How to access"                   "Name of resource"
              │                               │
    https://example.com/file.pdf    urn:isbn:0-486-27557-4

• URI: General term for resource identifiers
• URL: URI that specifies location + access method
• URN: URI that provides persistent name (location-independent)

10.9 Practical Tips for Medical Imaging Systems

10.9.1 DICOM Web URLs

# WADO (Web Access to DICOM Objects)
https://pacs.hospital.th/wado?requestType=WADO
  &studyUID=1.2.840.113619.2.55
  &seriesUID=1.2.840.113619.2.55.1
  &objectUID=1.2.840.113619.2.55.1.1

10.9.2 RESTful API for Radiology

GET  https://api.hospital.th/v1/studies           # List studies
GET  https://api.hospital.th/v1/studies/123       # Get specific study
POST https://api.hospital.th/v1/studies           # Create study
PUT  https://api.hospital.th/v1/studies/123       # Update study

10.9.3 Security Considerations

• Always use https:// for medical data (HIPAA/PDPA compliance)
• Never embed credentials in URLs
• Use proper authentication tokens (OAuth, JWT)
• Validate and sanitize query parameters to prevent injection attacks

10.10 Quick Reference

Component	Required	Sent to Server	Example
Scheme	Yes	Yes	https
Host	Yes	Yes	example.com
Port	No	Yes	:8443
Path	No	Yes	/api/data
Query	No	Yes	?id=123
Fragment	No	No	#top