This was a totally new one on me, a medieval trading network.
I think I've mentioned the Hanseatic League before on the Chrons. In a similar way to sknox, perhaps, it's been of interest to me for a long time. The idea of an international trading partnership, crossing borders, and with formalised trading agreements made at an almost diplomat level is rich for story mining.
They were able to force things through by legal and trade contracts, rather than simply sheer force of arms (although using local powers to fight their causes was also an option). Hansa ports and communities were dotted around the coast of Scotland and England, from Bristol to King's Lynn, on to Hull, Leith, Aberdeen and beyond; part of London was a Hanseatic kontor. From any of these places, you could deal directly with Bergen, or Danzig (modern day Gdansk), most of the Baltic coast, and then out to the Netherlands. And, each city along had its own trade network, meaning you could end up with goods from all over the known world.
Plus, there was a banking system, which allowed credit to be used in one port on collateral from another. Purely as a taster, and link to sources, Wikipedia has a decent page on the league.
On a similar note, it's interesting to note the number of sailors in British (and presumably other Northern European) sea towns, of the 18th Century and before, who originated from South Asia and Africa. There have always been trading routes; ships companies were often varied, needing new recruits and local knowledge of mariners who might stay on for a while; and wherever they settled, they either brought families with them, or made new ones.