IPv4 Subnet Calculator
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Subnet Results
What Is Subnetting?
Subnetting is the practice of dividing a single IP network into smaller, more manageable sub-networks (subnets). It allows network administrators to organise devices into logical groups, improve performance, and tighten security by isolating traffic between segments.
Every IPv4 address has two components: the network portion — shared by all devices on the same subnet — and the host portion — unique to each device within that subnet. The subnet mask defines exactly where the split occurs.
Understanding CIDR Notation
CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its subnet mask together — for example 192.168.1.0/24. The number after the slash is the prefix length: how many leading bits of the address are the network portion. A /24 prefix means 24 bits for the network and 8 bits for hosts, yielding 254 usable addresses.
| CIDR | Subnet Mask | Usable Hosts | Common Use |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 | Large ISP / Class A |
| /16 | 255.255.0.0 | 65,534 | Large enterprise / Class B |
| /24 | 255.255.255.0 | 254 | Small office / Class C |
| /25 | 255.255.255.128 | 126 | Half a Class C |
| /26 | 255.255.255.192 | 62 | Small department |
| /27 | 255.255.255.224 | 30 | Small team |
| /28 | 255.255.255.240 | 14 | Small office segment |
| /29 | 255.255.255.248 | 6 | Point-to-point + devices |
| /30 | 255.255.255.252 | 2 | Point-to-point link |
| /31 | 255.255.255.254 | 2 | Router-to-router (RFC 3021) |
| /32 | 255.255.255.255 | 1 | Single host / loopback route |
Key Terms Explained
- Network Address — The first address in a subnet. All host bits are zero. It identifies the subnet itself and cannot be assigned to a device.
- Broadcast Address — The last address in a subnet. All host bits are one. Packets sent here reach every device on the subnet. Also not assignable to individual devices.
- Subnet Mask — A 32-bit number with consecutive ones for the network bits and zeros for host bits (e.g. 255.255.255.0). ANDing an IP with its mask reveals the network address.
- Wildcard Mask — The bitwise inverse of the subnet mask (e.g. 0.0.0.255 for a /24). Used in access control lists (ACLs) and OSPF area definitions to specify which bits to ignore.
- Usable Hosts — The addresses between the network address and broadcast address. Calculated as 2(32−prefix) − 2.
- IP Class — A legacy classification (A, B, C) based on the first octet. Mostly replaced by CIDR, but still referenced in documentation and textbooks.