Use hp-makeuri to create “an url to your scanner”. For example:
hp-makeuri 192.168.2.22

Search the out for a line like:
SANE URI: hpaio:/net/Officejet_4620_series?ip=192.168.2.16

Now start simple-scan (or xsane) with the line above as parameter:
simple-scan hpaio:/net/Officejet_4620_series?ip=192.168.2.16