Subnet Calculator

CIDR (Classless Inter-Domain Routing) replaced the old class-based system in 1993. Instead of being stuck with /8, /16, or /24 boundaries, you can allocate any prefix length from /0 to /32.

The number after the slash is how many bits are the network portion. 192.168.1.0/24 means the first 24 bits identify the network, leaving 8 bits for hosts. That gives you 2⁸ - 2 = 254 usable addresses (minus network and broadcast).

For /31 and /32, the rules are different. /31 subnets (RFC 3021) have 2 addresses with no broadcast - used for point-to-point links. /32 is a single host route.

Leave room to grow. If you have 20 hosts, don't use a /27 (30 usable). Use a /26 (62 usable) or /25 (126). Renumbering a network is painful.

Cloud providers reserve addresses. AWS reserves 5 IPs per subnet (network, router, DNS, future, broadcast). A /28 that should have 14 hosts only gives you 11 on AWS.

Separate concerns with subnets. Put web servers, databases, and management on different subnets. It's easier to apply security groups and ACLs at the subnet level than per host.

Document everything. IP allocation gets messy fast. Track which subnets are assigned where, or you'll end up with overlapping ranges when you peer VPCs or set up VPN tunnels.

Subnetting splits a single network into smaller, isolated segments. Think of it like dividing a building into separate floors with locked doors between them. Each subnet gets its own address range, and traffic between subnets has to go through a router or firewall where you can apply rules.

For server infrastructure, this matters in three ways. Security: if an attacker compromises a web server on one subnet, they can't directly reach your database servers on another. Performance: broadcast traffic stays within its subnet, so a noisy service doesn't flood your entire network. Organization: it's much simpler to write firewall rules like "allow traffic from the web subnet to the database subnet on port 5432" than to manage individual IP rules.

A common pattern for a small production setup: put web servers on a /26 (62 hosts), databases on a /28 (14 hosts), and management/bastion hosts on a /29 (6 hosts). You can carve all three from a single /24 with room to spare.

Let's say you're assigned 10.0.1.0/24 for a project and need to split it into subnets for different roles:

This uses 168 addresses out of the 254 available in the /24, leaving 10.0.1.176/28 through 10.0.1.240/28 for future use. The key rule: each subnet's network address must be a multiple of its size. A /26 (64 addresses) starts at .0, .64, .128, or .192.