order allow,deny deny from 255.0.0.0 allow from all
This denies access to the IP 255.0.0.0.
255.0.0.0
order allow,deny deny from 123.45.6. allow from all
This denies access to all IPs in the range 123.45.6.0 to 123.45.6.255.
123.45.6.0
123.45.6.255