The boundary problem
Addresses identify hosts. Prefixes group networks.
A device needs an address, but a router also needs to know which part of that address describes the network and which part describes the host. The prefix length marks that boundary.
IPv4 addresses have 32 bits; IPv6 addresses have 128 bits. A prefix length counts leading bits, not devices.
In 192.168.10.24/24, the /24 prefix means the first 24 bits describe the network. The remaining bits identify a host inside it.
Build the model
Four useful ideas
Address
Identifies an interface or destination, such as 192.168.10.24.
Prefix
Marks how many leading bits describe the network, such as /24.
Subnet
Defines an address range and the boundary for traffic within that subnet.
IPv6
IPv6 uses a much larger address space. A /64 prefix is common for local IPv6 subnets, but other prefix lengths exist.
Mini experiment
Place the address
Keep the two example addresses fixed and change only the prefix. The count includes every address value, not just addresses assignable to interfaces.
An IPv4 range contains 2 to the power of (32 minus the prefix length) addresses: /24 gives 256, /26 gives 64, and /28 gives 16.
In these ordinary IPv4 subnets, the first address identifies the network and the last is reserved for broadcast; neither is normally assigned to an interface. Special /31 and /32 cases are outside this experiment.
The /24 prefix defines this IPv4 range.
Knowledge check
Test your subnet model
Answer five questions. The answer order changes each time.