Just like with any field name that has whitespaces in it, you'll have to enclose it in quotes. The following won't work on your system:
sourcetype=your_sourcetype | stats count by Asset IP Address
whereas this should work:
sourcetype=your_sourcetype | stats count by "Asset IP Address"
It's easy to see why: how should splunk know that you want to split by a field called <Asset IP Address>, and not split by the three fields <Asset>, <IP> and <Address>? You have to be specific here.
Same goes for your props.conf line, which has to be
FIELDALIAS-ipaddress = "Asset IP Address" AS ipaddress