Cisco Certified Support Technician (CCST) NetworkingNetworking FundamentalsMedium
A network administrator needs to assign a subnet to a new department with 60 hosts. The existing network uses a Class C IP address space (192.168.1.0/24). To accommodate the 60 hosts efficiently while minimizing wasted IP addresses, which subnet mask should be used?
- A255.255.255.224
- B255.255.255.240
- C255.255.255.192
- D255.255.255.128
Show answer & explanationAnswer & explanation
Correct answer: C. 255.255.255.192
A subnet mask of 255.255.255.192 corresponds to a /26 CIDR prefix. This allows for 2^(32-26) = 2^6 = 64 total IP addresses. After subtracting 2 for the network address and broadcast address, this leaves 62 usable host addresses, which is sufficient for 60 hosts while minimizing waste.
Why the other options are wrong
- A. 255.255.255.224 (/27) provides 30 usable host addresses (32 total - 2), which is insufficient for 60 hosts.
- B. 255.255.255.240 (/28) provides 14 usable host addresses (16 total - 2), which is insufficient for 60 hosts.
- D. 255.255.255.128 (/25) provides 126 usable host addresses (128 total - 2), which is more than needed and wastes addresses if smaller subnets are available.
Subnet Mask Calculation
Subnet mask calculation involves determining the number of bits to borrow from the host portion of an IP address to create subnets. This dictates the number of hosts per subnet and the number of available subnets.
- Formula: 2^h - 2 (where h = number of host bits) gives usable hosts.
- CIDR notation (/prefix) indicates the number of network bits.
- Each subnet requires a network address and a broadcast address, which are not usable by hosts.
Memory trick: Host Power: Two to the 'H' minus two, for subnet sanity.