.htaccessでアクセス制限

特定のIPのみアクセス拒否

order allow,deny
allow from all
deny from 123.45.67.89

特定のIPのみアクセス許可

order deny,allow
deny from all
allow from 123.45.67.89

IPアドレスだけでなくホスト名でも可

order allow,deny
allow from all
deny from example.com