Understanding Web Addresses

Web addresses are structured identifiers that help us locate and access resources on the internet. They follow specific formats and conventions to ensure consistency across the web.

What is a URI?

A Uniform Resource Identifier serves as a unique identifier for resources on the web. Think of it as a universal naming system that can either point to a location (like a street address) or simply name something (like a person’s name). URIs encompass both URLs and URNs, creating a comprehensive identification framework.

What is a URL?

A Uniform Resource Locator specifies the exact location of a resource and how to retrieve it. It’s like having both an address and directions to reach a destination. URLs typically use protocols like HTTP or FTP to define the access method.

Common examples:

• www.google.com
• foo:bar@w1.superman.com/very/long/path.html?p1=v1&p2=v2#more-details
• secured.com:443
• ftp.bogus.com/~some/path/to/a/file.txt

What is a URN?

A Uniform Resource Name provides a persistent identifier for resources regardless of their physical location. URNs use the urn: scheme and remain constant even if the resource moves. They function like ISBN numbers for books – the identifier stays the same no matter where you find the book.

Real-world examples:

• urn:isbn:0451450523 (Book identifier)
• urn:ietf:rfc:2648 (RFC document)
• urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66 (Unique identifier)

URI Structure Breakdown

Various URI formats:

• ftp://ftp.is.co.za/rfc/rfc1808.txt
• ldap://[2001:db8::7]/c=GB?objectClass?one
• news:comp.infosystems.www.servers.unix
• tel:+1-816-555-1212
• telnet://192.0.2.16:80/

Understanding User Information

The userinfo segment contains credentials for authentication, combining username and optionally a password. It appears before the @ symbol and hostname. Modern security practices often prompt for passwords separately rather than embedding them in URLs.

Format patterns:

• ftp://username:password@host.com/
• ftp://username@host.com/

The Authority Component

Authority encompasses the userinfo, hostname, and port sections. While userinfo and port are optional, the hostname is essential. When no port is specified, default ports apply automatically – port 80 for HTTP connections and 443 for HTTPS.

Authority variations:

• username:password@host.com/
• subdomain.domain.com
• www.superaddress.com:8080

Fragment Identifiers

Fragments (or hash values) direct users to specific sections within a resource. Following the # symbol, they act as bookmarks pointing to particular content areas without requiring a new page load.

Fragment examples:

• www.foo.bar/listings.html#section-2

Path Structure

Paths represent a hierarchical structure using segments divided by forward slashes (/). Similar to folder organization on a computer, each segment narrows down the resource location within the server’s structure.

Path examples:

• www.foo.bar/segment1/segment2/some-resource.html
• www.foo.bar/image-2.html?w=100&h=50
• ftp.foo.bar/~john/doe

Query Strings Explained

Query strings transmit additional data to the server using key-value pairs. Each pair connects with an equals sign (=), while ampersands (&) separate multiple pairs. Query strings begin after the question mark (?) character.

Query string patterns:

• www.foo.bar/image.jpg?height=150&width=100
• secured.com:443/resource.html?id=6e8bc430-9c3a-11d9-9669-0800200c9a66#some-header