If your use-case is from ssh'ing in a remote box, this may be simpler:

  #!/bin/sh
  # example from `who - u`:
  #nick     pts/0  2011-07-20 20:44 . 7856 (xxxxxxx.xxxxxxxx.internode.on.net)

  who -u | awk 'BEGIN {FS="[p.]"} /internode/ {print gensub("-",".","g",$6); exit}'

You might need to play around with gensub(...). Sometime `who` reports the IP as xxx-xxx-xxx-xxx and sometimes as xxx.xxx.xxx.xxx