Subnet Mask Notation Calculator

About this interactive

A subnet mask tells a device where the network part of an address ends: a one for every network bit, then a zero for every host bit, 32 bits in all. The default subnet mask draws that line at the class boundary: 255.0.0.0 (/8) for Class A, 255.255.0.0 (/16) for Class B, 255.255.255.0 (/24) for Class C. A custom subnet mask draws it somewhere else, usually further right, after you borrow host bits. The prefix length, written after a slash in CIDR notation, is just the count of ones: the class default plus the bits borrowed. 140.12.0.0 is Class B, so borrowing 8 bits gives /24, and 210.15.3.0 is Class C, so borrowing 4 gives /28. To write a prefix as a mask, fill whole octets with 255 and put the leftover bits in the next octet, where the ones fill from the left: 1 bit is 128, 2 are 192, then 224, 240, 248, 252, 254 and 255. /13 is 255.248.0.0, /18 is 255.255.192.0 and /28 is 255.255.255.240. To go back, count the ones in each octet and add them up. Type masks in dotted decimal, prefixes as a number with no slash, and binary as 8 digits with no spaces. Anything you miss shows the answer and how to work it out. Seven of the twenty-one questions are drawn each run.

What you'll learn

Give the default subnet mask of a Class A, B or C address, in dotted decimal and as a prefix length
Convert a custom subnet mask between bits borrowed, prefix length, dotted decimal and binary

Aligned to

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing.

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting

Cisco CCST Networking

2.2 Identify IPv4 addresses and subnet formats

Key terms

Subnet Mask: A 32-bit number that divides an IP address into network and host portions.
Default Subnet Mask: A subnet mask that draws the network/host boundary at the classful boundary of an IP address class, such as 255.0.0.0 for a Class A address.
Custom Subnet Mask: A subnet mask that draws the network/host boundary at a point other than the classful boundary, allowing flexible division of network and host bits.
Prefix Length: The number of consecutive 1-bits in a subnet mask, written after a slash (e.g., /24), that defines how many bits of an IP address represent the network portion versus the host portion. A /24 prefix means the first 24 bits identify the network, leaving 8 bits for host addresses.
Classless Inter-Domain Routing: A method for allocating IP addresses and expressing routing prefixes that replaces the older classful addressing scheme by using variable-length prefix notation (e.g., /24). CIDR enables more efficient use of IPv4 address space and reduces the size of internet routing tables through route aggregation.
Bit Borrowing: The practice of taking bits from the host portion of a classful default subnet mask to create additional network subdivisions, extending the prefix length.
Octet: One of the four 8-bit sections of an IPv4 address, each separated by a dot in dotted decimal notation and representing a decimal value from 0 to 255. For example, in 192.168.1.1, the four octets are 192, 168, 1, and 1.

Topics

Subnet Mask Default Subnet Mask Custom Subnet Mask Cidr Notation Interactive Calculation

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