code: func GetSubnetMask(o dhcp4.Options) SubnetMask {
  	v := o.Get(dhcp4.OptionSubnetMask)
  	if v == nil {
  		return nil
  	}
  	var s SubnetMask
  	if err := (&s).UnmarshalBinary(v); err != nil {
  		return nil
  	}
  	return s
  }
docs: // GetSubnetMask returns the subnet mask of `o`.
  //
  // This returns nil if the option is not present or did not contain a valid
  // value.
  //
  // The subnet mask option is defined by RFC 2132, Section 3.3.