Usage: GENERAL SET DNS [on|off]

If you turn this off then client addresses will appear as IP addresses rather than as host domains.

/as general set dns on
/as general set dns off

You will usually want this on; otherwise everyone will appear to come from numeric addresses. There's nothing wrong with everyone coming from numeric addresses, but it doesn't look quite as nice. Whichever you choose, you should do consistently, so that people will usually match their access lists.