Subnet Calculator

Subnetting divides a single IP network into smaller, isolated segments. Each subnet has its own network address, broadcast address, and range of usable host addresses. For ISPs, subnetting is fundamental to network architecture — it determines how subscriber traffic is segmented, how management VLANs are isolated, and how efficiently public and private address space is utilized.

CIDR (Classless Inter-Domain Routing) notation, defined in RFC 4632, replaced the old Class A/B/C system in 1993, enabling flexible prefix lengths from /0 to /32. A /24 gives you 254 usable hosts, a /30 gives you 2, and a /20 gives you 4,094 — allowing ISPs to allocate exactly the right amount of address space for each purpose.

Every subnet calculation boils down to bitwise operations on the IP address and subnet mask:

• Network address = IP AND subnet mask
• Broadcast address = network OR wildcard mask
• Usable hosts = 2^(32 − prefix length) − 2
• Wildcard mask = NOT subnet mask (255.255.255.255 − mask)

The binary representation in the calculator above shows exactly how these operations work bit-by-bit. The 1-bits in the mask identify the network portion, while the 0-bits identify the host portion.

Effective subnet planning is critical for ISP operations. Common allocation patterns:

• /31 or /30 — Point-to-point links between routers and OLTs
• /28 to /26 — Management VLANs for network devices
• /24 to /22 — Subscriber segments per VLAN or area
• /20 or larger — CGNAT pools (RFC 6598, 100.64.0.0/10)

Always over-provision slightly — renumbering a subnet is far more disruptive than allocating a few extra addresses upfront. Pair your IP address planning with switch and port monitoring to track utilization and detect oversubscribed segments before they affect subscribers.