IP Subnet Calculator

This subnet calculator can help you with the following tasks:

• Identify subnet parameters for a given IP address and subnet mask (or CIDR prefix).
• Display subnet parameters for a number of consecutive subnets of the specified length.
• Customize subnet configuration within a given IP address range by combining or splitting subnets.

Start by entering IP address either in a dot-decimal notation (e.g., 10.1.1.5) or in CIDR notation (e.g., 10.1.1.5/24). In the latter case, the provided prefix length will define the Subnet mask value. This calculator supports prefix length in the range /2 .. /31.

Specify the number of Subnets to display and then click Submit to show the results. You can further customize subnet configuration by splitting or combining subnets with the help of the "divide" ("÷") and "add" ("+") buttons in the right-most column.

Click Copy link to copy to the clipboard the link to this page with the entered IP and subnet values. Note that subnet customization will not be included in the link.

Use Copy table to send the content of the subnet table to the clipboard.

To view IP and subnet information in binary format use our IP/Mask Visual Calculator.

Classless Inter-Domain Routing (CIDR) is a method used to allocate and specify Internet Protocol (IP) addresses and IP routing on the Internet. CIDR replaces the traditional system of IP address classes (Class A, B, and C) with a more flexible and scalable approach. The key idea behind CIDR is to allow the allocation of IP addresses in subnets with variable-length prefixes, breaking away from the fixed-length subnet masks used in the class-based addressing.

Key components and concepts of CIDR:

Prefix Length: In CIDR notation, an IP address is followed by a slash ("/") and a number representing the length of the prefix or subnet mask. This prefix length indicates the number of bits used for the network portion of the address. For example, "192.168.1.0/24" represents a subnet with a prefix length of 24 bits and a subnet mask of 255.255.255.0.

Address Aggregation: CIDR enables aggregation of IP addresses into larger blocks, reducing the size of routing tables in the Internet's backbone routers. Instead of advertising individual IP addresses or small blocks, ISPs can advertise larger CIDR blocks, which simplifies routing and reduces the amount of routing information exchanged between routers.

Efficient Address Allocation: CIDR allows for more efficient allocation of IP addresses by enabling organizations to use smaller subnets with the number of addresses they require, rather than being constrained by predefined classful boundaries.

Variable Length Subnet Masking (VLSM) is a technique used in CIDR networks to optimize address allocation by allowing different subnets to have subnet masks of varying lengths. With VLSM, network administrators can allocate IP addresses with more granularity, assigning larger subnets to areas with more hosts and smaller subnets to areas with fewer hosts. This allows for more efficient use of address space compared to traditional fixed-length subnetting.

Number of hosts on a /31 subnet:

Generally, within a subnet, two host addresses - all-zeros and all-ones are reserved as network address and broadcast, respectively. The same approach applied to a /31 subnet with only two possible addresses in the host identifier would leave no usable addresses for host assignment. RFC3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links allows assignment of the all-zeros and all-ones IP addresses to hosts on point-to-point links.

See Also
IP Address and Subnet Mask Visual Calculator
IPv6 Calculator
Subnet Mask Table