Subnetting is guaranteed to appear on CompTIA Network+ N10-009, typically as 3–5 questions requiring you to calculate network addresses, usable host ranges, broadcast addresses, and subnet masks from CIDR notation. Network+ subnetting questions are less complex than CCNA — you are not configuring devices — but you must quickly determine valid host ranges, identify whether two IPs are in the same subnet, and understand VLSM (Variable Length Subnet Masking) for efficient address use.
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A subnet mask tells a device which part of an IP address is the network portion and which is the host portion. Written as dotted decimal (255.255.255.0) or CIDR notation (/24). The mask's 1 bits identify the network portion; 0 bits identify the host portion. The CIDR number equals the count of consecutive 1 bits from left.
Common masks to memorize: /8 = 255.0.0.0 (Class A default), /16 = 255.255.0.0 (Class B default), /24 = 255.255.255.0 (Class C default). For subnetting: /25 = 255.255.255.128 (128 addresses, 126 hosts), /26 = 255.255.255.192 (64 addresses, 62 hosts), /27 = 255.255.255.224 (32 addresses, 30 hosts), /28 = 255.255.255.240 (16 addresses, 14 hosts), /29 = 255.255.255.248 (8 addresses, 6 hosts), /30 = 255.255.255.252 (4 addresses, 2 hosts — point-to-point links).
Given an IP and CIDR (e.g., 192.168.10.65/26), determine: Block size = 256 − subnet mask last octet. For /26, mask = 255.255.255.192, block size = 256−192 = 64. Network addresses are multiples of the block size: 0, 64, 128, 192. The host 65 falls in the 64 block. Network address: 192.168.10.64. Broadcast: 192.168.10.127 (next block − 1). Usable hosts: 192.168.10.65 to 192.168.10.126 (126 hosts).
Number of subnets from a base network: 2^(borrowed bits). Number of usable hosts per subnet: 2^(host bits) − 2 (subtract network and broadcast addresses). The −2 is always required for standard unicast subnets.
VLSM (Variable Length Subnet Masking) allows different subnets within the same major network to use different mask lengths — larger subnets for large segments, /30 for point-to-point WAN links, /32 for loopback interfaces. VLSM maximizes address efficiency.
RFC 1918 defines three private address ranges not routed on the public internet: 10.0.0.0/8 (Class A — 16.7M addresses), 172.16.0.0/12 (Class B — 172.16.0.0 to 172.31.255.255, ~1M addresses), 192.168.0.0/16 (Class C — 65,536 addresses). Devices using private IPs access the internet via NAT.
Special addresses: 127.0.0.1 = loopback (localhost). 169.254.0.0/16 = APIPA (Automatic Private IP Addressing) — assigned when DHCP fails. 0.0.0.0 = this network/any address. 255.255.255.255 = limited broadcast. Class D: 224.0.0.0–239.255.255.255 = multicast. Class E: 240.0.0.0–255.255.255.254 = reserved/experimental.
| CIDR | Subnet Mask | Addresses | Usable Hosts | Use Case |
|---|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 | Standard LAN |
| /25 | 255.255.255.128 | 128 | 126 | Half a /24 |
| /26 | 255.255.255.192 | 64 | 62 | Small dept |
| /27 | 255.255.255.224 | 32 | 30 | Small team |
| /28 | 255.255.255.240 | 16 | 14 | Very small segment |
| /29 | 255.255.255.248 | 8 | 6 | Tiny segment |
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point WAN |
| /32 | 255.255.255.255 | 1 | 0 (host route) | Loopback, host route |
A /24 network has 256 usable hosts
A /24 has 256 total addresses but only 254 usable hosts — the network address (.0) and broadcast address (.255) cannot be assigned to devices
Any IP address in the 172.x.x.x range is private
Only 172.16.0.0 through 172.31.255.255 is private (RFC 1918). Addresses from 172.0.x.x to 172.15.x.x and 172.32.x.x and above are public
APIPA means the device has no network connectivity
APIPA (169.254.x.x) means DHCP failed — the device can communicate with other APIPA-addressed devices on the same link, but cannot reach the internet or corporate servers. It indicates a DHCP problem, not a physical connectivity failure
These questions are representative of what you will see on Network+ exams. The correct answer and explanation are shown immediately below each question.
A host has the IP address 192.168.5.100 with a subnet mask of 255.255.255.192. What is the broadcast address for this subnet?
Explanation: Mask 255.255.255.192 = /26. Block size = 256−192 = 64. Subnets: .0, .64, .128, .192. Host .100 falls in the .64 block. Network address = 192.168.5.64. Broadcast = next block start − 1 = 192.168.5.128 − 1 = 192.168.5.127. Usable range: 192.168.5.65 – 192.168.5.126.
A technician notices a workstation has the IP address 169.254.47.12. What does this indicate?
Explanation: 169.254.x.x is the APIPA (Automatic Private IP Addressing) range, automatically assigned by Windows when a DHCP server cannot be reached. This indicates DHCP failure — the DHCP server may be down, unreachable, or the DHCP scope is exhausted. The technician should troubleshoot DHCP connectivity.
Use the 'magic number' method: (1) Find the last non-255 octet of the mask. (2) Block size = 256 − that octet value. (3) Network addresses = 0, block-size, 2×block-size, etc. (4) Find which block contains the host IP. (5) Network = start of block, broadcast = next block − 1, hosts = network+1 to broadcast−1. Practice until this is automatic — the exam is timed.
No. Network+ subnetting questions test identification of network addresses, broadcast addresses, usable ranges, and whether two IPs are in the same subnet — all from CIDR notation. CCNA additionally tests wildcard masks, advanced VLSM design, and configuring routes. Network+ subnetting is conceptual identification, not device configuration.
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