Metal - organic coordination networks (MOCNs) have attracted wide interest because they provide a novel route towards porous materials that may find applications in molecular recognition, catalysis, gas storage and separation(1,2). The so-called rational design principle - synthesis of materials with predictable structures and properties - has been explored using appropriate organic molecular linkers connecting to metal nodes to control pore size and functionality of open coordination networks(3-9). Here we demonstrate the fabrication of surface-supported MOCNs comprising tailored pore sizes and chemical functionality by the modular assembly of polytopic organic carboxylate linker molecules and iron atoms on a Cu(100) surface under ultra-high-vacuum conditions. These arrays provide versatile templates for the handling and organization of functional species at the nanoscale, as is demonstrated by their use to accommodate C-60 guest molecules. Temperature-controlled studies reveal, at the single-molecule level, how pore size and chemical functionality determine the host - guest interactions.