  The topological structures of the Internet and the Web have received
considerable attention. However, there has been little research on the
topological properties of individual web sites. In this paper, we consider
whether web sites (as opposed to the entire Web) exhibit structural
similarities. To do so, we exhaustively crawled 18 web sites as diverse as
governmental departments, commercial companies and university departments in
different countries. These web sites consisted of as little as a few thousand
pages to millions of pages. Statistical analysis of these 18 sites revealed
that the internal link structure of the web sites are significantly different
when measured with first and second-order topological properties, i.e.
properties based on the connectivity of an individual or a pairs of nodes.
However, examination of a third-order topological property that consider the
connectivity between three nodes that form a triangle, revealed a strong
correspondence across web sites, suggestive of an invariant. Comparison with
the Web, the AS Internet, and a citation network, showed that this third-order
property is not shared across other types of networks. Nor is the property
exhibited in generative network models such as that of Barabasi and Albert.
