In Apache 2.4 this is a bit different from what you may be used from Apache 2.2 ; so here it goes:
Apache 2.4 htaccess example (without using proxy services like cloudflare)
<RequireAll>
Require ip 10.20.30.40
</RequireAll>
Now if you use a proxy service like cloudflare / incapsula things may be different as Apache does get the IP of those providers and you really do not want everyone to access the site; in that case you can use the X-Forwarded-For which contains the 'real' user IP.
Apache 2.4 htaccess example with cloudflare
SetEnvIF X-Forwarded-For "10.20.30.40" AllowIP
<RequireAll>
Require env AllowIP
</RequireAll>
(change the IP 10.20.30.40 with the IP you need; you can add multiple lines to provide access to more IP's; you can also add a complete network in CIDR notation).
