During the last three decades the Internet has experienced fascinating evolution, both exponential growth in traffic and rapid expansion in topology. The size of the Internet becomes enormous, yet the network is very `small in the sense that it is extremely efficient to route data packets across the global Internet. This paper provides a brief review on three fundamental properties of the Internet topology at the autonomous systems (AS) level. Firstly the Internet has a power-law degree distribution, which means the majority of nodes on the Internet AS graph have small numbers of links, whereas a few nodes have very large numbers of links. Secondly the Internet exhibits a property called disassortative mixing, which means poorly-connected nodes tend to link with well-connected nodes, and vice versa. Thirdly the best-connected nodes, or the rich nodes, are tightly interconnected with each other forming a rich-club. We explain that it is these structural properties that make the global Internet so small.
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