To identify a logical topology of each virtual network among logical topologies of a plurality of virtual networks configured in physical devices. The control circuit comprises a modulation circuit and an oscillation circuit to modulate the switching frequency of switching signals for power saving. The sealing element includes a mounting surface adapted to sealingly mate against a loading dock, an inner sealing surface at a transverse angle relative to the mounting surface, an outer sealing surface opposing the inner sealing surface and at a transverse angle relative to the mounting surface, and an impact-absorbing surface approximately parallel to the mounting surface. The credential contains a lower layer address, a network layer address, a domain name, and a network port identifier. The method of forming characters for microprint includes determining for every image character a size font taking into account the rules to create small prints, rasterizing a character into a bitmap, skeletonizing the bitmap, reformatting the bitmap maintaining a character legibility, storing the bitmap, and forming halftoning cells from the reformatted bitmap. Thereby, it is possible to prevent the layer having an aperture from being destroyed.