/26 subnet: mask 255.255.255.192, 62 usable hosts

Reading /26

Two bits of the last octet join the network part, giving mask 255.255.255.192 and wildcard 0.0.0.63. Those two bits select one of four 64-address blocks.

2^6 = 64 addresses, 62 usable after network and broadcast. 10.0.0.0/26 spans 10.0.0.1 to 10.0.0.62, broadcast 10.0.0.63. The next block starts at 10.0.0.64.

Good fits

Sixty-odd hosts suits a rack of servers, a management network for switches and PDUs, or a small branch office. It also suits per-availability-zone subnets in a modest VPC where each zone runs a handful of instances.

After AWS or Azure take their five reserved addresses, a /26 subnet keeps 59. Managed services that drop network interfaces into your subnet (load balancers, NAT gateways, Lambda in a VPC) also draw from that pool, so leave headroom.

Splitting

A /26 halves into two /27s or four /28s, and two aligned /26s form a /25. Mixing sizes inside one /24 is fine as long as each block is aligned: a /25 at .0, a /26 at .128 and two /27s at .192 and .224 fill it exactly.

Configuring a /26

For 10.0.0.0/26 put the gateway on 10.0.0.1 and keep hosts at or below 10.0.0.62; 10.0.0.63 is the broadcast and must not be assigned. A frequent typo is giving a host .64, which is the network address of the next /26, not the 64th host of this one.

When a /26 per zone feels tight, count the managed interfaces first. A load balancer, a NAT gateway, a few VPC endpoints and a database instance can take a dozen addresses before the first application server starts.

Splitting a /24 into four /26s across three availability zones leaves one block spare, which is a tidy home for a later fourth zone or a dedicated endpoints subnet without touching the existing three.