You said MAC addresses and the one's I've seen have dashes. Do you mean IP addresses with decimals? No matter. The procedure is the same.
First, split the address into as many columns as needed using text to column feature like you did before. Now, using the empty column to the right or left of them (insert a column if needed), use the CONCATENATE function to put them all back together inserting a decimal where needed. It goes like this:
(1) Your numbers with spaces are all in column A to start with.
(2) Use text to columns to split those numbers to as many columns as you need.
(3) Use the empty column to the right or left (insert one if needed) of where all the resulting columns of data reside.
(4) Now, assuming you're on row 1 column D, the formula would look something like this (modify as necessary):
=concatenate(A1, ".", B1, ".", C1)
(5) Copy that formula down each row to the bottom of the data.
(6) Once you've varified that everything is correctly copied and formatted (the correct numbers are represented), select all of the cells in that column. Do not click the column header to select. Be sure just the filled cells are selected and COPY them (ctrl+c or right click and choose copy).
(7) Place the cursor back at the top of that column, use Edit/Paste Special, choose Values from the resulting menu, then click OK. This will replace the formula which is only displaying the numbers with the actual numbers themselves formatted as text and not numerical. The contents of every cell will begin with an appostrophe though. So be aware of that if you need to use that data in other applications.
(8) Delete the old split out columns as you no longer need them.
Hope this helps.
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