Cisco CCNA (200-301)Network FundamentalsMedium
A network administrator needs to assign IP addresses to 15 hosts on a new LAN segment. They have been allocated the network 192.168.10.0/28. How many usable host IP addresses are available in this subnet?
- A30
- B62
- C16
- D14
Show answer & explanationAnswer & explanation
Correct answer: D. 14
A /28 subnet mask means there are 28 network bits and (32 - 28) = 4 host bits. The total number of IP addresses is 2^4 = 16. Subtracting 2 for the network address and broadcast address leaves 16 - 2 = 14 usable host IP addresses.
Why the other options are wrong
- A. This would be the usable hosts for a /27 (2^5 - 2 = 30).
- B. This would be the usable hosts for a /26 (2^6 - 2 = 62).
- C. This is the total number of IP addresses in the subnet, not the usable host addresses.
Usable Host IP Addresses Calculation
The number of usable host IP addresses in a subnet is calculated by subtracting two (for the network and broadcast addresses) from the total number of addresses in the subnet.
- Total addresses = 2^(32 - CIDR prefix).
- Network address is the first address in the subnet.
- Broadcast address is the last address in the subnet.
Memory trick: Hosts are like seats: count them all, then remove the first and last.