Endpoint address for loading map

Hello OSM Community,

I’m working over application which will be used in private network so I should allow access for it. I am using


As far as I understand I’m getting the maps as a list of images by providing base URL, filename ending and size in pixels (OnlineTileSourceBase.java). In the TileSourceFactory.java class I have method

 public static final OnlineTileSourceBase MAPNIK = new XYTileSource("Mapnik",
			0, 19, 256, ".png", new String[] {
					"https://c.tile.openstreetmap.org/" },"© OpenStreetMap contributors",

So should I allow access to urls https://a(b,c).tile.openstreetmap.org ?

What does the three letters mean ? Are there more of them (should it be *.tile.openstreetmpa.org), are they just them three ?

All answers are appreciated.

Have a nice day!