Now my question is how to do it. Is the best way to go once through the input file, sort it by address and then do it as before? But I think that this will have a slow performance. Here you see only a small example. The file can be really huge (e.g. 200 MB)

If the file is huge, I'd use sort(1) (that is, the external command) to sort the file. One may want to use grep to filter out unwanted lines (those not starting with S3). You may want to use a filter to put the address part into a column for sorting.

One of the advantages is that sort(1) doesn't easily run out of memory (although, with modern computers, 200MB should be ok) - it will resort to using temp files if it doesn't have enough memory to do it all in-memory.