Subnet calculator

How to work out a subnet by hand

Take 192.168.1.130/26. The prefix says the first 26 of the address's 32 bits name the network, and the other 6 number a device on it.

11000000.10101000.00000001.10000010

1. The mask is 26 ones then 6 zeros: 255.255.255.192. The wildcard mask is its inverse, 0.0.0.63, the form access lists and OSPF use.
2. The block size is 2 to the power of the host bits: 2⁶ = 64. Subnets of this size start at every multiple of 64 in the last octet: 0, 64, 128, 192.
3. The network address is the multiple of 64 at or below 130: 192.168.1.128. In binary, it is the address with every host bit set to 0 (the address AND the mask).
4. The broadcast address is the next block's start less one: 192.168.1.191, every host bit set to 1.
5. The hosts are everything between: 192.168.1.129 to 192.168.1.190, 62 addresses (64 less the network and broadcast addresses). A /31 and a /32 are the exceptions: a point-to-point link and a single address keep every address usable.

To drill the method on random addresses, with each answer checked, try subnetting practice.

Every prefix, its mask and its size

From /0, the whole IPv4 space, to /32, one address. Usable hosts leave out the network and broadcast addresses, except in a /31 and a /32.