The number after the slash counts network bits. For IPv4, /26 leaves six address bits and therefore 64 total addresses. The subnet begins on a boundary determined by those bits; it does not necessarily begin at the address you typed.
Read the prefix before the address
An IPv4 address contains 32 bits. In 192.168.10.70/26, the first 26 identify the network and the remaining six identify positions within it. The matching mask is 255.255.255.192. Six remaining bits create 2^6 = 64 addresses. Ordinary multi-host IPv4 subnets reserve the first address for the network and the last for broadcast.
A mask does not say which addresses your DHCP server will hand out. The DHCP pool may occupy only part of the subnet, leaving room for router interfaces, reservations, and other infrastructure.
Work through 192.168.10.70/26
The final-octet boundaries for /26 are 0, 64, 128, and 192. Address 70 sits in the block beginning at 64.
| Network | 192.168.10.64/26 |
|---|---|
| Last address / broadcast | 192.168.10.127 |
| Traditional host range | 192.168.10.65–192.168.10.126 |
| Addresses | 64 total; 62 traditional hosts |
Choosing 192.168.10.70 as the starting address of a 64-address block would produce the wrong boundary. Calculate the network before writing a firewall rule or creating a DHCP scope.
The /31, /32, and IPv6 exceptions
A /31 is useful for IPv4 point-to-point links: both addresses can identify endpoints. A /32 is one address, often used for a host route. Do not apply the usual subtract-two formula to these cases. IPv6 has no broadcast address and its address count is not an IPv4-style usable-host pool.
A shorter prefix covers a larger range. A /24 contains four aligned /26 blocks. Combining routes is only exact when the component networks fill an aligned parent block; two arbitrary adjacent-looking ranges may require several CIDRs.
Plan a subnet you can operate
- Count current devices, growth, infrastructure, and address reservations.
- Check for overlap with other LANs, VPN networks, and routed sites.
- Calculate boundaries and test both the first and last intended host.
- Document the gateway, DHCP pool, DNS settings, and firewall policy separately.
For example, a small office needing 45 device addresses could fit a /26 today, but future growth and fixed devices may justify a /25. Pick the size from requirements rather than memorizing a familiar mask. Changing a live subnet also changes routing and client configuration, so schedule the migration and retain a way to reach the router.
Put it into practice
References & further reading
Protocol standards and official documentation supporting this guide. Device menus and provider policies vary; use the documentation for your equipment when applying a change.