"#" in IRI references

I have a question about IRI references.

Given that "#" can appear unescaped anywhere in an IRI reference, how does a
parser distinguish the ifragment from the absoluteIRI or relativeIRI?

Sorry if this has been brought up before. I feel like I must be missing
something obvious here.

    - Mike
____________________________________________________________________________
   mike j. brown                   |  xml/xslt: http://skew.org/xml/
   denver/boulder, colorado, usa   |  resume: http://skew.org/~mike/resume/