How to calculate an IPv4 subnet, with a worked example

Every IPv4 subnet question comes down to one operation: a bitwise AND between the address and the subnet mask. Once that clicks, the network address, broadcast address and host range follow directly. Here it is worked through for 192.168.10.77/26.

1. Turn the prefix into a mask

/26 means the first 26 of the 32 bits are network bits:

11111111.11111111.11111111.11000000  =  255.255.255.192

Only the last octet is interesting here. The 6 host bits give a block size of 2⁶ = 64 addresses.

2. AND the address with the mask

77  = 01001101
192 = 11000000
AND = 01000000  =  64

So the network address is 192.168.10.64. A shortcut that gives the same answer: blocks of 64 start at 0, 64, 128 and 192, and 77 falls in the block that starts at 64.

3. Read off the rest

• Broadcast address: last address of the block, 64 + 64 − 1 = 192.168.10.127
• Usable hosts: 192.168.10.65 to 192.168.10.126 — 64 − 2 = 62 hosts
• Wildcard mask (used in ACLs): 255.255.255.255 − 255.255.255.192 = 0.0.0.63

The general formula for usable hosts is 2^{(32 − prefix)} − 2, because the first address identifies the network and the last is the broadcast. Two exceptions: a /31 has two usable addresses for point-to-point links (RFC 3021), and a /32 is a single host.

Check your answer

Enter 192.168.10.77 and prefix 26 in the IP Subnet Calculator. It shows the same network (192.168.10.64), broadcast (192.168.10.127), host range, 62 usable hosts, and the mask in binary (11111111 11111111 11111111 11000000) so you can compare it with your working.

Splitting a /24 into smaller subnets

Borrowing 2 bits from the host part of 192.168.10.0/24 gives 2² = 4 subnets of /26, each with 62 usable hosts:

192.168.10.0/26     hosts .1   – .62    broadcast .63
192.168.10.64/26    hosts .65  – .126   broadcast .127
192.168.10.128/26   hosts .129 – .190   broadcast .191
192.168.10.192/26   hosts .193 – .254   broadcast .255

To convert individual octets between binary, decimal and hex while practising, use the number base converter.