CompTIA Network+ (N10-009)Networking ConceptsHard
A network administrator configures a firewall rule using a wildcard mask of 0.0.0.15 to match a range of addresses for an ACL. How many host addresses does this wildcard mask cover?
- A32
- B16
- C8
- D64
Show answer & explanationAnswer & explanation
Correct answer: B. 16
A wildcard mask is the inverse of a subnet mask; 0.0.0.15 in binary is 00001111, meaning the last 4 bits are 'don't care' bits, covering 2^4 = 16 addresses (equivalent to a /28 subnet with 16 total addresses).
Why the other options are wrong
- A. 32 addresses would correspond to a wildcard mask of 0.0.0.31 (/27).
- C. 8 addresses would correspond to a wildcard mask of 0.0.0.7 (/29).
- D. 64 addresses would correspond to a wildcard mask of 0.0.0.63 (/26).
Wildcard Mask
A wildcard mask is the inverse of a subnet mask, used in ACLs and routing protocols to specify which bits must match and which are ignored.
- Wildcard mask = 255.255.255.255 minus subnet mask
- 0 bits mean 'must match', 1 bits mean 'don't care'
- Number of covered addresses = 2^(number of 1-bits) in the wildcard octet
Memory trick: Wildcard is the subnet mask's evil twin — flip the bits to find the range.