NFS setup

[n dhert]

I try to set up NFS between two freebsd-8.1 systems.
Both were - at installation time - set up to act as NFS server and NFS
client
so both have in their /etc/rc.conf
nfs_client_enable="YES"
nfs_server_enable="YES"
rpcbind_enable="YES"

all the services are running (checked on both systems):
# ps -jaxw | grep nfs
root         882     1   882   882    0 Is    ??    0:00.04 nfsd: master
(nfsd)
root         884   882   882   882    0 S     ??    0:00.00 nfsd: server
(nfsd)
# ps -jaxw | grep mountd
root         880     1   880   880    0 Is    ??    0:00.00 /usr/sbin/mountd
-r
# ps -jaxw | grep rpcbind
root         747     1   747   747    0 Ss    ??    0:00.01
/usr/sbin/rpcbind
the NFS server (server.subdomain.topdomain) has
server# cat /etc/exports
/home -alldirs -ro client.subdomain.topdomain
server# showmount -e
Exports list on localhost:
/home           client.subdomain.topdomain
server# cat /etc/hosts
XXX.YYY.ZZZ.CCC         client.subdomain.topdomain

the NFS client (client.subdomain.topdomain) has
client# cat /etc/hosts
XXX.YYY.ZZZ.SSS         server.subdomain.topdomain
and has a mount-point /mnt with nothing mounted on it yet (empty dir)

client# mount server.subdomain.topdomain:/home /mnt
   (or client# mount XXX.YYY.ZZZ.SSS:/home /mnt)
hangs, and after trying for about a minute, responds
[tcp] server.subdomain.topdomain:/home: RPCPROG_NFS: RPC: Port mapper
failure - RPC: Timed out

what is missing to make this work?