IPv4 Subnet Calculator

A subnet mask is a 32-bit number that divides an IPv4 address into a network and host portion. It helps determine which part of the address identifies the network and which part identifies devices (hosts) within that network.

An IPv4 address is a 32-bit binary number, usually written in dotted decimal notation (e.g., 192.168.1.10). A subnet mask works by masking part of this address to define the network.

• IP Address: 192.168.1.10
• Subnet Mask: 255.255.255.0

• IP Address: 11000000.10101000.00000001.00001010
• Subnet Mask:11111111.11111111.11111111.00000000

Commonly, people believe a /31 or /32 offers no usable hosts in a traditional sense because:

• /31: Would normally have 2 IPs (one network address and one broadcast).
• /32: Would logically have just 1 IP, often considered a host route or loopback.

However, /31 addresses can be used for point-to-point links(RFC 3021), effectively allowing both IPs to be assigned to the two endpoints.

/32 addresses are frequently used for loopback interfaces, router IDs, or point-to-multipoint links in specific setups (e.g., OSPF, BGP). So while they don’t offer traditional “host ranges,” they are still very much usable in routing scenarios.

Number of subnets: Count how many bits you borrowed from the host portion to create subnets. If you borrowed n bits from the default mask, then the number of subnets is 2ⁿ.

Number of hosts per subnet: Count how many bits remain in the host portion. If you have h bits left for hosts, the total IP addresses in that subnet is 2ʰ, and the usable hosts (traditionally) is 2ʰ - 2 (subtracting network and broadcast addresses).

In modern contexts (especially with /31 and /32 in specialized scenarios), remember that the "usable host formula" can change. But for typical subnets, 2ʰ - 2 is still a solid rule.

Private addresses: These are reserved for internal networks and are not routable on the public internet. Common private ranges are:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

Public addresses: These are globally unique and are routable on the internet.

Most organizations use private addresses internally and rely on NAT (Network Address Translation)to provide internet access for internal devices.

Memorize the common dotted-decimal to CIDR conversions:

• 255.0.0.0 = /8
• 255.255.0.0 = /16
• 255.255.255.0 = /24
• 255.255.255.128 = /25, etc.

Use quick binary math:

•  0  = 00000000 (0 bit)
• 128 = 10000000 (1 bit)
• 192 = 11000000 (2 bits)
• 224 = 11100000 (3 bits)
• 240 = 11110000 (4 bits)
• 248 = 11111000 (5 bits)
• 252 = 11111100 (6 bits)
• 254 = 11111110 (7 bits)

Practice with a cheat sheet or use a subnet calculator until these mappings become second nature.