How to determine whether the configured wildcard mask of the IP network segment is correct?
1. In a subnet mask, a sequence of 1s designate that part as being the IP network identifier, and a block of 0s following the 1s designate that part as being the host identifier. It can be inferred that the last number of a subnet mask of a network segment must be even. For example 192.168.1.0 255.255.255.128, in decimal notation.2. To obtain the wildcard mask, one can just use the result of 255 subtracted by the given octet of the original mask. Because the last digit of the mask is always an odd number (5), the last digit of a correct wildcard mask must be an odd number based on the formula of "An odd number � An even number = An odd number". For example, the wildcard mask of 255.255.255.128 is 0.0.0.127.
3. Conclusion: The last number of a subnet mask of a network segment must be an even one, and its wildcard mask must be an odd number.
Service anomalies caused by the incorrect wildcard mask are usually serious, which cause some functions to become invalid or forwarding of some services to fail. This method can help the engineer quickly locate the faulty configuration of the wildcard mask and solve the problem.