cat file |   sed "......" > file2

HOST2
{
     aaa bbb
     cccc ddd
    ...
}

     # replace only between lines  1  and  20 
      1 , 20  s/Johnson/White/g

     # replace everywhere EXCEPT between lines  1  and  20 
      1 , 20  !s/Johnson/White/g

     # replace only between words "from" and "until". Note the
     # use of \<....\> as word boundary markers in GNU sed.
     /from/,/until/ { s/\<red\>/magenta/g; s/\<blue\>/cyan/g; }

     # replace only from the words "ENDNOTES:" to the end of file
     /ENDNOTES:/,$ { s/Schaff/Herzog/g; s/Kraft/Ebbing/g; }

cat SystemDefinition.cfg | sed "/HOST2/,/}/ {s,^,\!,g;} "