IPv4 Subnet Calculator

Split this block

Each extra prefix bit halves the block. Pick a target prefix to see the resulting subnets.

CIDR reference

How subnetting works

An IPv4 address is a single 32-bit number; the dots are only formatting. A prefix length splits those 32 bits into a network part and a host part. Every host on a link shares the network part, which is what lets a router decide, from one AND operation, whether a packet is local or has to be forwarded.

The netmask is that split written as a bitmask: /24 is 24 ones followed by 8 zeros, printed 255.255.255.0. Because the ones have to be contiguous and left-aligned, a mask and a prefix length carry exactly the same information - which is why CIDR notation replaced dotted netmasks for most purposes in 1993.

Two addresses in every block are reserved. All-zeros in the host part names the network itself; all-ones is the directed broadcast. That is where “254 usable hosts in a /24” comes from, and it is why a /30 - four addresses - gives you only two usable ones. The exceptions are /31, where RFC 3021 dispenses with the broadcast for point-to-point links, and /32, a single host route.

Wildcard masks and ACLs

A wildcard mask is the bitwise NOT of the netmask, and Cisco ACLs and OSPF network statements want it rather than the netmask. A /24 becomes 0.0.0.255. The rule is the same one every bitmask follows: a 0 in the wildcard means “this bit must match”, a 1 means “don't care”. Writing the netmask where a wildcard is expected silently matches the wrong traffic, which is one of the more expensive typos in networking.

Private and reserved ranges

The 172.16.0.0/12 boundary catches people out: 172.16 through 172.31 are private, but 172.32.0.0 onwards is public address space belonging to somebody else.

Code Examples

import ipaddress

net = ipaddress.ip_network("192.168.1.0/24")

net.network_address   # 192.168.1.0
net.broadcast_address # 192.168.1.255
net.netmask           # 255.255.255.0
net.hostmask          # 0.0.0.255
net.num_addresses     # 256
list(net.hosts())[0]  # 192.168.1.1

# split into four /26s
[str(s) for s in net.subnets(prefixlen_diff=2)]

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