What is a Subnet Mask?
Learn what a subnet mask is, how it splits network and host bits, and how to calculate usable hosts — with interview Q&A.
Expected Interview Answer
A subnet mask is a 32-bit value paired with an IPv4 address that tells a device which bits of the address identify the network and which bits identify the specific host, so it can determine whether a destination is on the local network or must be reached through a router.
Written in the same dotted-decimal form as an IP address (e.g., 255.255.255.0) or as a CIDR prefix length (e.g., /24), the subnet mask has contiguous 1 bits covering the network portion and contiguous 0 bits covering the host portion; a bitwise AND of the IP address and the mask yields the network address. For example, with IP 192.168.1.10 and mask 255.255.255.0 (/24), the network is 192.168.1.0 and the host portion is the final octet, giving 254 usable host addresses (256 total minus the network and broadcast addresses). When a device needs to send a packet, it applies its own subnet mask to both its own IP and the destination IP: if they land in the same network, the packet is delivered directly on the local segment (resolved via ARP); if not, the device forwards the packet to its default gateway for routing. Longer masks (like /26 or /30) create smaller subnets with fewer hosts, which is the basis of subnetting and VLSM (Variable Length Subnet Masking) for efficient address allocation.
- Lets a device split an IP address into network and host portions
- Determines whether a destination is local or needs a gateway
- Enables subnetting to size networks appropriately (fewer wasted addresses)
- Maps directly to CIDR prefix notation for concise configuration
AI Mentor Explanation
A subnet mask is like the rule that says the first two digits of a seat code identify the stand and the last two digits identify the exact seat within it — an usher applies that same rule to any ticket to instantly know whether a fan belongs in this stand or needs directing elsewhere. Applying the rule consistently is what lets an usher sort thousands of fans quickly instead of checking every seat individually. A device applying its subnet mask to an IP address works exactly the same way, splitting it into network and host portions. Getting the digit-split rule wrong would send fans to the wrong stand, just as a wrong mask misroutes packets.
Step-by-Step Explanation
Step 1
Pair mask with IP
A subnet mask (e.g., 255.255.255.0 or /24) is assigned alongside a device's IP address.
Step 2
Bitwise AND
The device ANDs its IP with the mask to determine its own network address.
Step 3
Compare destinations
For any destination IP, the same mask determines if it falls in the same network (local) or a different one (remote).
Step 4
Route or deliver locally
Local destinations are resolved via ARP and delivered directly; remote destinations are sent to the default gateway.
What Interviewer Expects
- Correct definition: separates network bits from host bits in an IP address
- Can convert between dotted-decimal mask and CIDR prefix notation
- Can calculate network address, broadcast address, and usable hosts for a given mask
- Explains how the mask decides local delivery vs default gateway routing
Common Mistakes
- Forgetting to subtract network and broadcast addresses when counting usable hosts
- Confusing subnet mask with a wildcard mask (used in ACLs, which is inverted)
- Assuming all networks default to a /24 (255.255.255.0)
- Not knowing the mask determines local-vs-gateway forwarding decisions
Best Answer (HR Friendly)
“A subnet mask is like the rule that tells you which part of a street address is the neighborhood and which part is the house number — it lets a computer figure out whether another device is on the same local network or needs to be reached through a router. Without it, a device would not know whether to deliver data directly or send it out to the wider network.”
Code Example
# Show this machine’s IP address and subnet mask (Linux)
ip -4 addr show eth0
# inet 192.168.1.10/24 brd 192.168.1.255 scope global eth0
# Convert /24 to dotted-decimal and inspect network/broadcast/hosts
ipcalc 192.168.1.10/24
# Netmask: 255.255.255.0 = 24
# Network: 192.168.1.0/24
# HostMin: 192.168.1.1
# HostMax: 192.168.1.254
# Broadcast: 192.168.1.255Follow-up Questions
- How do you calculate the number of usable hosts from a subnet mask?
- What is the difference between a subnet mask and a wildcard mask?
- How does a device decide to use its default gateway based on the mask?
- What is VLSM and why does it need variable-length masks?
MCQ Practice
1. What does a subnet mask primarily determine?
A subnet mask separates the network portion of an IP address from the host portion.
2. What is the dotted-decimal form of a /24 subnet mask?
A /24 prefix corresponds to 255.255.255.0, leaving the last octet for host addresses.
3. How many usable host addresses does a /24 subnet have?
A /24 has 256 total addresses; subtracting the network and broadcast addresses leaves 254 usable hosts.
Flash Cards
What is a subnet mask? — A 32-bit value that separates the network portion of an IP address from the host portion.
/24 in dotted decimal? — 255.255.255.0.
How is the network address computed? — By bitwise ANDing the IP address with the subnet mask.
What decides local delivery vs gateway routing? — Comparing the destination IP against the local network using the subnet mask.