I was trying to find out how many portholes there are on the ISS, and where they are located, but no luck, so thought I'd try here. I found info for the Russian components, but nothing for the US. I read James Oberg saying there were quite a few, of good optical properties, but where are they? What direction do they face? Any help appreciated!
On the Russian Segment, there are six nadir-facing portholes in the Service Module (SM), as well as one porthole on each of the two EVA hatches of both the Docking Compartment-1 (DC-1) and Mini Research Module-2 (MRM-2). All four of these portholes are "clocked" at plus/minus 45 degrees to the positive/negative velocity vector - so DC-1 has portholes facing port-forward and starboard-aft, while MRM-2 has portholes facing starboard-forward and port-aft.
In addition, Soyuzes offer three portholes when docked - via two side-looking portholes in the Descent Module, and one forward-looking porthole in the Orbital Module. Due to their specific clocking when docked, a Soyuz docked to MRM-2 has portholes facing port, starboard, and nadir, while a Soyuz docked to MRM-1 has portholes facing starboard-forward, port-aft, and zenith.
On the US Segment, in addition to the obvious Cupola, there is one large nadir-facing optical-quality porthole on the US lab (which is used exclusively for Earth observation payloads via the WORF rack), and two port-facing portholes on the JPM.
In addition, every USOS CBM hatch has a porthole, however the portholes on unused ports/hatches are covered with an external flap (called a CBCS flap), which can only be uncovered via EVA if the port is unused. The only portholes on unused CBM ports that are currently NOT covered with a CBCS flap are the Node 2 nadir and zenith ports - so these are the only CBM hatch portholes that the crew can "see" out of (and there isn't really much to see out of the Node 2 zenith porthole - except the blackness of space).