Subnet Mask Cheat Sheet

About the Subnet Mask Cheat Sheet

A complete subnet mask cheat sheet: all 33 IPv4 prefix lengths from /0 to /32 with the dotted subnet mask, the wildcard (inverse) mask used in Cisco ACLs, the hexadecimal and binary forms, the number of addresses and usable hosts, and how many /24 networks each one equals. Below it, the common IPv6 prefix sizes — /48, /56, /64, /127 and others — with what they are normally used for and the source for each.

Search with whatever you have: /26, 255.255.255.192, 0.0.0.63, 0xffffffc0 or 50 hosts (it picks the smallest subnet that fits). Select any prefix — or type a CIDR such as 192.168.1.77/24 — to see its details and apply it to an address: the network, first and last usable address and broadcast address appear at once. Printed, both tables fit on one A4 or US Letter page at the browser’s default margins.

Common uses

• Looking up a mask while configuring a router, firewall, server or cloud subnet.
• Converting between CIDR notation, dotted masks and Cisco wildcard masks.
• Studying subnetting for CCNA, Network+ or a networking course — and printing a reference sheet.
• Checking how many hosts a subnet holds before handing it out.

Reading the table

• Prefix: the number of leading 1-bits in the mask (CIDR notation, RFC 4632).
• Subnet mask: the same mask in dotted decimal, as most operating systems and routers expect it.
• Wildcard: the mask inverted (255 − each number). Cisco access lists and OSPF network statements use it.
• Hex: the mask as one hexadecimal number; macOS and BSD ifconfig show masks this way, for example netmask 0xffffff00.
• Addresses / usable hosts: a /n has 2^(32 − n) addresses; hosts can use all but the network and broadcast addresses, so 2^(32 − n) − 2. A /31 is the exception for point-to-point links, where both addresses are usable (RFC 3021); a /32 is one host.
• /24s: how many /24 networks (256 addresses each) the prefix equals — handy when ranges are discussed in “class C” sized blocks. The figures match the table in RFC 1878.

Subnetting shortcuts

• Each extra prefix bit halves the block: a /24 has 256 addresses, a /25 has 128, a /26 has 64.
• Block size = 256 − the last non-zero number of the mask. For 255.255.255.224 that is 256 − 224 = 32, so subnets start at .0, .32, .64, .96 and so on.
• The broadcast address is the next subnet’s start minus one: in 192.168.1.64/27 it is 192.168.1.95.
• The default masks of the old address classes were /8 (class A), /16 (class B) and /24 (class C). CIDR replaced classes in 1993 (RFC 1519, now RFC 4632), but the names are still used.

IPv6 prefix sizes

IPv6 subnets are almost always /64: stateless address autoconfiguration (SLAAC) on Ethernet and Wi-Fi needs a 64-bit interface identifier (RFC 4862, RFC 2464). Sites receive a larger prefix and split it into /64s: RIPE-690 recommends a /48 for business customers and at least a /56 for residential ones. Links between routers may use a /127 (RFC 6164). IPv6 has no broadcast address, so no addresses are lost to it.

Limitations

• The host counts follow the standard rule (network and broadcast reserved). Some cloud providers reserve more addresses per subnet — check their documentation.
• Applying a prefix works for IPv4 addresses; use the IPv6 table as a size reference.

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