mount: unknown filesystem type 'nfs'

[Dan Fulbright <dan+debian-user-list@dan.tulsa.ok.us>]

When I try to mount an NFS filesystem, I get this error:

mount: unknown filesystem type 'nfs'

Here's the mount command I'm using on host2:

mount host1.domain.com:/tmp /mnt

On host1.domain.com, I have this in /etc/exports:

/tmp host2.domain.com(ro,sync)

I tried using all IP addresses instead of names, but with the same 
results. The mount(8) man page says that it includes built-in NFS support.

I'm running Sarge. I'm somewhat new to Debian, but have been using Linux 
for many years, so if I need to provide more information, just let me know.

Thanks in advance.

--df