Cant mount a directory on NFS server

I have shared direcroty hirarchy named /var/ftp/pub on my nfs
ealier it was working fine. i wa sable to acess it form client. But
now its giving an error when i tried to mount it from client
with command :-

mount //server1:/var/ftp/pub /mnt

it gives error message

mount: can't get address for server1

I tried rpc from both sides it workes well.

Both computer pings eac other

shpumount shows all directoreis & client name

pls help