Subnet Mask Table

A subnet mask is a fundamental concept in TCP/IP networking that defines how an IPv4 address is partitioned into a network identifier and a host identifier. Every IPv4 address is 32 bits long, and the subnet mask determines which bits belong to the network and which bits can be assigned to individual devices. This division allows the internet to route traffic efficiently by grouping addresses into hierarchical networks rather than treating each address as an isolated endpoint.

CIDR notation, introduced in 1993, simplified subnet representation by using a forward slash followed by the number of network bits. For example, /24 means the first 24 bits are the network portion, equivalent to the subnet mask 255.255.255.0. CIDR replaced the older classful networking system and enabled more efficient address allocation through variable-length subnet masking. You can explore how specific IP addresses break down into subnets using our Subnet Calculator.

IPv4 Address Classes

Before CIDR, IPv4 addresses were divided into five classes (A through E). Class A addresses (1.0.0.0 to 126.0.0.0) used an 8-bit network prefix and were allocated to very large networks. Class B addresses (128.0.0.0 to 191.255.0.0) used a 16-bit prefix for medium-sized organizations. Class C addresses (192.0.0.0 to 223.255.255.0) used a 24-bit prefix for smaller networks. Classes D and E were reserved for multicast and experimental purposes, respectively.

Private IP Ranges (RFC 1918)

Not all IP addresses are meant for the public internet. RFC 1918 defines three private address ranges that organizations can use internally without registering them with regional internet registries. These addresses are translated to public IPs through Network Address Translation (NAT). The ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Home routers typically assign addresses from the 192.168.x.x range to local devices.

Broadcast & Loopback Addresses

Every subnet reserves two special addresses: the network address (all host bits set to 0) and the broadcast address (all host bits set to 1). The broadcast address allows a single packet to reach every host on the local segment. The 127.0.0.0/8 loopback range is reserved for local testing and inter-process communication, with 127.0.0.1 being the most commonly used address to refer to the local machine itself.

Calculating Usable Hosts

The number of usable host addresses in a subnet is calculated with the formula 2^(32-CIDR) - 2. The subtraction of 2 accounts for the network and broadcast addresses. For example, a /24 subnet supports 254 hosts, a /16 supports 65,534 hosts, and a /30 supports 2 hosts—commonly used for point-to-point router links. Understanding this formula is essential for network design and IP address planning.

• Wikipedia — Subnet Mask
• RFC 1918 — Address Allocation for Private Internets
• RFC 950 — Internet Standard Subnetting Procedure
• RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links

If you are working with network configurations, you may also find our Subnet Calculator useful for computing network ranges, broadcast addresses, and host counts from any IP and CIDR. For diagnosing domain issues, try our DNS Lookup tool to query A, MX, TXT, and other DNS records.