IP Subnet Calculator

CIDR (Classless Inter-Domain Routing) notation is written as an IP address followed by a slash and a number such as 192.168.1.0/24. The number after the slash represents the number of consecutive 1 bits in the subnet mask, defining how many bits are allocated to the network portion versus the host portion. This replaced the traditional classful addressing system with fixed subnet masks of /8, /16, and /24 for classes A, B, and C respectively. CIDR allows for variable-length subnet masks (VLSM), giving network administrators flexibility to create subnets of any size that precisely match their needs without wasting IP addresses.

The formula for calculating usable hosts is 2 raised to the power of (total bits minus network bits) minus 2. For a /24 subnet in IPv4: 2^(32-24) - 2 = 2^8 - 2 = 256 - 2 = 254 usable hosts. For a /26 subnet: 2^(32-26) - 2 = 2^6 - 2 = 64 - 2 = 62 usable hosts. We subtract 2 because the network address (all host bits set to 0) and the broadcast address (all host bits set to 1) are reserved and cannot be assigned to individual hosts. The exception is /31 subnets used for point-to-point links where both addresses are usable per RFC 3021.

A subnet mask is a 32-bit number for IPv4 that separates an IP address into two parts: the network portion and the host portion. The bits set to 1 in the subnet mask correspond to the network portion, and the bits set to 0 correspond to the host portion. For example, the subnet mask 255.255.255.0 (/24) in binary is 11111111.11111111.11111111.00000000, meaning the first 24 bits identify the network and the last 8 bits identify hosts within that network. When a device needs to determine if another IP is on the same local network, it performs a bitwise AND between its own IP and subnet mask, does the same for the destination IP, and compares the results.

Private IP addresses are RFC 1918 defined ranges that are not routable on the public internet. For IPv4, the three private ranges are: 10.0.0.0/8 supporting over 16 million hosts, 172.16.0.0/12 supporting about 1 million hosts, and 192.168.0.0/16 supporting about 65,000 hosts. The 100.64.0.0/10 range (RFC 6598) is used for Carrier-Grade NAT. Use private IPs for all internal network addressing, home networks, corporate LANs, and VPN tunnels. Your router or firewall performs NAT to map internal private addresses to a public IP for internet communication, conserving public IPv4 space and adding security.

VLSM (Variable Length Subnet Masking) allows different subnet masks within the same major network, allocating subnets of different sizes based on actual needs. Without VLSM, a point-to-point WAN link needing only 2 addresses would consume the same size subnet (254 hosts for /24) as a large LAN segment. With VLSM, you can assign /30 for point-to-point links (2 usable hosts), /27 for small departments (30 hosts), and /23 for larger segments (510 hosts). VLSM is implemented by subnetting a subnet, starting with the largest subnets first, maximizing address utilization and conserving the limited IPv4 address space.