IPv4 Subnet Masks and CIDR Prefixes Explained
One page per IPv4 prefix length worth knowing, with the mask, the wildcard mask, the host count and the real-world use. The subnet calculator is on every page, so you can put your own address in and get the network, broadcast and usable range for it straight away.
Every page here embeds the IP Subnet Calculator. Enter any IPv4 address with a prefix or a dotted mask and get the network address, broadcast address, wildcard mask, usable host range, address class, and the hex and binary forms.
21 pages in this section.
Large blocks, /0 to /16
What a /0 subnet is: the default route
A /0 uses the mask 0.0.0.0 and gives 4,294,967,294 usable addresses. What it is for and where it goes wrong.
What a /4 subnet is: the multicast block
A /4 uses the mask 240.0.0.0 and gives 268,435,454 usable addresses. What it is for and where it goes wrong.
What a /8 subnet is: a full class A block
A /8 uses the mask 255.0.0.0 and gives 16,777,214 usable addresses. What it is for and where it goes wrong.
What a /10 subnet is: the carrier-grade NAT block
A /10 uses the mask 255.192.0.0 and gives 4,194,302 usable addresses. What it is for and where it goes wrong.
What a /12 subnet is: the 172.16 private block
A /12 uses the mask 255.240.0.0 and gives 1,048,574 usable addresses. What it is for and where it goes wrong.
What a /14 subnet is: a four-way /16 aggregate
A /14 uses the mask 255.252.0.0 and gives 262,142 usable addresses. What it is for and where it goes wrong.
What a /16 subnet is: a full class B block
A /16 uses the mask 255.255.0.0 and gives 65,534 usable addresses. What it is for and where it goes wrong.
Site blocks, /18 to /24
What a /18 subnet is: a quarter of a /16
A /18 uses the mask 255.255.192.0 and gives 16,382 usable addresses. What it is for and where it goes wrong.
What a /20 subnet is: sixteen /24s
A /20 uses the mask 255.255.240.0 and gives 4,094 usable addresses. What it is for and where it goes wrong.
What a /21 subnet is: eight /24s
A /21 uses the mask 255.255.248.0 and gives 2,046 usable addresses. What it is for and where it goes wrong.
What a /22 subnet is: four /24s
A /22 uses the mask 255.255.252.0 and gives 1,022 usable addresses. What it is for and where it goes wrong.
What a /23 subnet is: two /24s
A /23 uses the mask 255.255.254.0 and gives 510 usable addresses. What it is for and where it goes wrong.
What a /24 subnet is: the classic LAN subnet
A /24 uses the mask 255.255.255.0 and gives 254 usable addresses. What it is for and where it goes wrong.
Small subnets, /25 to /32
What a /25 subnet is: half a /24
A /25 uses the mask 255.255.255.128 and gives 126 usable addresses. What it is for and where it goes wrong.
What a /26 subnet is: a quarter of a /24
A /26 uses the mask 255.255.255.192 and gives 62 usable addresses. What it is for and where it goes wrong.
What a /27 subnet is: an eighth of a /24
A /27 uses the mask 255.255.255.224 and gives 30 usable addresses. What it is for and where it goes wrong.
What a /28 subnet is: sixteen addresses
A /28 uses the mask 255.255.255.240 and gives 14 usable addresses. What it is for and where it goes wrong.
What a /29 subnet is: eight addresses
A /29 uses the mask 255.255.255.248 and gives 6 usable addresses. What it is for and where it goes wrong.
What a /30 subnet is: a point-to-point link
A /30 uses the mask 255.255.255.252 and gives 2 usable addresses. What it is for and where it goes wrong.
What a /31 subnet is: a point-to-point link with no waste
A /31 uses the mask 255.255.255.254 and gives 2 usable addresses. What it is for and where it goes wrong.
What a /32 subnet is: a single host
A /32 uses the mask 255.255.255.255 and gives 1 usable addresses. What it is for and where it goes wrong.