mac = "98:4f:ee:00:0c:3a"

x = mac.split(':')
x.insert(3, 'ff')
x.insert(4, 'fe')
x[0] = (x[0].to_i(16) ^ 0b00000010).to_s(16)
x.insert(0, 'fe')
x.insert(1, '80')
x = x.each_slice(2).map{|x|x.join('')}.join(':')
x.insert(4, ":")

print(x)