g1smd - 10:11 am on Feb 9, 2013 (gmt 0)
www.example.com/p173820082-acme-model-34684-widget is the product page. It's a single URL for the product.
It is linked to from:
where each of those category pages shows its own breadcrumb navigation.
Rather than show breadcrumb trails at the top of the product page, instead a box on the right says
etc, and those link back to the relevant category pages.
If you used cookies, it would also be possible to show a personalised breadcrumb trail of how the user got to this page; and omit it for searchengines and users without cookies.
There's also a set of standard navigation across the top of the page. The bottom of the page links to a larger number of categories and sub-categories.
Additionally, the reviews are at
www.example.com/r173820082-acme-model-34684-widget and detailed product specs are at
Extensionless URLs are a key part of the process - makes the rewrite rules much easier to craft. The system is also self-checking, and this is much easier when the URL contains a unique ID. You'll see a lot of systems where the unique ID is at the end of the URL path. I prefer it at the beginning. Were you to request the truncated URL
www.example.com/p173820082-acme-mo the system would redirect you to the correct product URL. This also allows the page URL slug to be amended to correct typos in product names. The system auto-updates search engine listings and redirects users continuing to request the typo'd URL.
The most important point is that you "gotta have a plan" for the URL structure and the URL format. It needs to be documented in detail before any coding starts.