be more specific with the fixed math

_posts/2014-07-14-ipv6.md

@@ -122,7 +122,7 @@ Yes. An attacker could port-scan your /64 and try to find the non-temporary
 address, but keep in mind that finding that one address out of 2<sup>64</sup>
 addresses would mean that you have to port-scan 4 billion traditional v4
 internets per attack target (good luck) or randomly guessing with an average
-chance of 1:(2<sup>64</sup>/2) (also good luck).
+chance of 1:2<sup>63</sup> (also good luck).
 
 Even then a personal firewall could block all unsolicited packets from
 non-local prefixes to provide even more security.