NetworkingNetwork+

Subnetting for CompTIA Network+ N10-009

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.

11 min

3 sections · 7 exam key points

2 practice questions

Subnet Mask Fundamentals

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).

Calculating Subnet Information

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.

Private IP Address Ranges

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.

Common CIDR Subnet Reference

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

Key exam facts — Network+

Usable hosts = 2^(host bits) − 2; subtract 1 for network address, 1 for broadcast
Block size = 256 − last octet of subnet mask
Network addresses are multiples of the block size
APIPA range: 169.254.0.0/16 — assigned when DHCP fails
Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
/30 = 2 usable hosts — used for point-to-point WAN links
VLSM allows different mask lengths within the same major network

Common exam traps

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

Practice questions — Subnetting

These questions are representative of what you will see on Network+ exams. The correct answer and explanation are shown immediately below each question.

Q1.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?

A.192.168.5.127

B.192.168.5.191

C.192.168.5.255

D.192.168.5.63

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.

Q2.A technician notices a workstation has the IP address 169.254.47.12. What does this indicate?

A.The workstation is using a VPN

B.The workstation failed to receive a DHCP address

C.The IP address is a public routable address

D.The workstation is using IPv6 transition

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.

Frequently asked questions — Subnetting

How do I quickly calculate subnets on the Network+ exam?

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.

Does Network+ test subnetting to the same depth as CCNA?

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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