sort -un gives just the last line

[Jan Stary]

I have a list of IP addresses.
Sorting them with sort -n works as expected.
Sorting them with sort -u works as expected.

$ sort -u /tmp/list
173.194.64.26
173.194.64.27
173.194.65.26
173.194.65.27
173.194.66.26
173.194.66.27
173.194.67.26
173.194.67.27
173.194.69.26
173.194.70.26
173.194.70.27
173.194.71.26
173.194.71.27
173.194.78.26
173.194.78.27

But:

$ sort -u -n /tmp/list
173.194.70.27

This is just the last one in the file.
Am I missing something obvious?

        Jan