  We present a halo model prediction of the image separation distribution of
strong lenses. Our model takes account of the subhalo population, which has
been ignored in previous studies, as well as the conventional halo population.
Halos and subhalos are linked to central and satellite galaxies by adopting an
universal scaling relation between masses of (sub-)halos and luminosities of
galaxies. Our model predicts that 10%-20% of lenses should be caused by the
subhalo population. The fraction of lensing by satellite galaxies (subhalos)
peaks at ~1'' and decreases rapidly with increasing image separations. We
compute fractions of lenses which lie in groups and clusters, and find them to
be ~14% and ~4%, respectively: Nearly half of such lenses are expected to be
produced by satellite galaxies, rather than central parts of halos. We also
study mass distributions of lensing halos and find that even at image
separations of ~3'' the deviation of lens mass distributions from isothermal
profiles is large: At or beyond ~3'' image separations are enhanced
significantly by surrounding halos. Our model prediction agrees reasonably well
with observed image separation distributions from galaxy to cluster scales.
