The TCP/IP Guide - URL Relative Syntax and Base URLs

The description above probably seems confusing, but relative URLs are actually fairly easy to understand, because they are interpreted in a rather “common sense” way. You simply take the base URL and the relative URL, and you substitute whatever information is in the relative URL for the appropriate information in the base URL to get the resulting equivalent absolute reference. In so doing, you must “drop” any elements that are more specific than the ones being replaced.

What do I mean by “more specific”? Well, most URLs can be considered to move from “most general” to “most specific” in terms of the location they specify. As you go from left to right, you go through the host name, then high-level directories, subdirectories, the file name, and optionally, parameters/query/fragment applied to the file name. If a relative URL specifies a new file name, it replaces the file name in the base URL, and any parameters/query/fragment are dropped as they no longer have meaning given that the file name has changed. If the relative URL changes the host name, the entire directory structure, file name and everything else “to the right” of the host name “goes away”, replaced with any that might have been included in the new host name specification.

This is hard to explain in words but easy to understand with a few examples. Let's assume we start with the following explicit base URL:

Table 224 shows some example of relative URLs and how they would be interpreted.

**Table 224: Relative URL Specifications and Absolute Equivalents**
Relative URL	Equivalent Absolute URL	Explanation
#bookmark2	http://site.net/dir1/subdir1/file1?query1#bookmark2	The URL is the same except that the bookmark is different. This can be used to reference different places in the same document in HTML. (Technically, the URL has not changed here, since the “fragment” (bookmark) is not part of the actual URL. A Web browser given a new bookmark name will usually not try to re-access the resource.)
?query2	http://site.net/dir1/subdir1/file1?query2	The same file but with a different query string. Note that the bookmark reference from the base URL is “stripped off”.
file2	http://site.net/dir1/subdir1/file2	Here we have referred to a file using the name “file2”, which replaces “file1” in the base URL. Here both the query and bookmark are removed.
/file2	http://site.net/file2	Since a single slash was included, this means “file2” is in the root directory; this relative URL replaces the entire <url-path> of the base URL.
..	http://site.net/dir1/	The pair of dots refers to the parent directory of the one in the base URL. Since the directory in the base URL is “dir1/subdir1”, this refers to “dir1/”.
../file2	http://site.net/dir1/file2	Specifies that we should go up to the parent directory to find the file “file2” in “dir1”.
../subdir2/file2	http://site.net/dir1/subdir2/file2	Go up one directory with “..”, then enter the subdirectory “subdir2” to find “file2”.
../../dir2/subdir2/file2	http://site.net/dir2/subdir2/file2	Same thing as above but going up two directory levels, then down through “dir2” and “subdir2” to find “file2”.
//file2	http://file2	Two slashes means that “file2” replaces the host name, causing everything to the right of the host name to be stripped. This is probably not what was intended, and shows how important it is to watch those slashes. J
//www.newsite.net/otherfile.htm	http://www.newsite.net/otherfile.htm	Here everything but the scheme has been replaced. (In practice this form of relative URL is not that common—the scheme is usually included if the site name is specified, for completeness.)
file2?query2#bookmark2	http://site.net/dir1/subdir1/file2?query2#bookmark2	Here we replace the file name, query name and bookmark name.
ftp://differentsite.net/whatever	ftp://differentsite.net/whatever	Using a new scheme forces the URL to be interpreted as absolute.

URL Schemes (Applications / Access Methods) and Scheme-Specific Syntaxes

URL Length and Complexity Issues

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