Names for types of URLs

Question

1. Absolute
   http://www.example.com/images/icons.png

2. Relative
   ../images/icons.png

3. ???
   /images/icons.png

4. ???
   //www.example.com/images/icons.png

Do URL types 3 and 4 have names? One place I've seen type 4 being used is on Slashdot.

Answer 1

1. Absolute http://www.example.com/images/icons.png
2. Document-Relative ../images/icons.png
3. Root-Relative /images/icons.png
4. Protocol-Relative //www.example.com/images/icons.png

For #4, I've also often called them "Protocol-Agnostic"

Answer 2

Type 1 is just a "URI" (sometimes called an "absolute URI").

For types 2, 3 and 4 the definitive answers are in RFC 3986, section 4.2.

They are all "relative references", but according to the RFC are qualified thus:

• ../images/icons.png - "relative path reference"
• /images/icons.png - "absolute path reference"
• //.../icons.png - "network path reference"

The latter is often used if you want to specify a URL containing a domain name, but where you want the protocol to match the protocol used to access the current resource. For example, if your images are downloaded from a CDN, you could use this to default to https if the current page was also downloaded via https, thus preventing the warning about including non-secure resources in a secure page.