Your "desired result" is identical to your original file.

One solution:

local $/ = "\n}\n";
while (<>) {
   s/
       ^(fixed-address [^\n]*\n)
       (.*)
       ^(hardware ethernet [^\n]*\n)
   /$3$2$1/xsm;
   print;
}
[download]

Or if the lines are always adjacent when they're in the wrong order,

local $/ = "\n}\n";
while (<>) {
   s/
       ^
       (fixed-address .*\n)
       (hardware ethernet .*\n)
   /$2$1/xm;
   print;
}
[download]