What is a subnet and what does a mask like 255.255.255.0 mean?
Published on 2026-10-20 by frfrand
Subredes y máscaras CIDR explicadas desde cero: qué significa /24, cómo se separan redes y cómo evitar errores al configurar servidores.
A subnet is a part of a computer network that uses a defined range of IP addresses. The subnet mask is used to distinguish which part of an address identifies the network and which part can vary between devices.
The definition seems very technical until you see 192.168.1.0/24 or 255.255.255.0 in a server's configuration. Then the inevitable question arises: what do those numbers mean and why do they matter?
The neighborhood example
Imagine an enormous city divided into neighborhoods. A street and a number serve to find a building, but it also matters knowing which neighborhood it belongs to.
A network can be organized into subnets for similar reasons: to distinguish groups, apply different rules, and make it easier for routers to know where to send data.
It does not mean that each subnet necessarily requires a different building or physical router. Today many networks are virtual.
The IP address does not tell the whole story
Let's take the example 192.168.1.25. By itself it is an IPv4 address, but it is not enough to determine which is the local network: the prefix or mask is missing.
If that device is configured in 192.168.1.0/24, the number /24 indicates that the first 24 bits of the address identify the network part.
As IPv4 uses 32-bit addresses, 8 bits remain to vary within that subnet: in total there are 256 possible addresses.
In a traditional IPv4 subnet like this, 192.168.1.0 usually represents the network address and 192.168.1.255 the broadcast address. That leaves 254 addresses normally usable for devices; some platforms reserve more for their own services.
Where does 255.255.255.0 come from?
The mask 255.255.255.0 represents the same boundary as /24 in IPv4. The initial 255s correspond to groups of eight network bits, and the last 0 leaves a group of eight bits for devices.
You don't need to learn to convert binary mentally to get started. What matters is recognizing that /24 is not a port, not a password, and not the maximum number of devices allowed by any router.
It is an abbreviated way of indicating how much the network portion occupies within the address.
What happens if I change from /24 to /26?
A subnet with prefix /26 uses 26 bits for the network, so 6 bits remain for addresses within each block: 64 addresses per subnet, normally 62 usable in the usual IPv4 model.
For example, the block 192.168.1.0/24 can be divided into four /26 blocks:
192.168.1.0/26192.168.1.64/26192.168.1.128/26192.168.1.192/26
Notice that the starting addresses increase by 64 each time. It's not a coincidence: the block size changed.
If you mix an IP from one block with the mask from the other, a device may try to reach another as if they were on the same network when, in reality, it needs to go through a router.
Do two devices on the same subnet communicate without a router?
In a typical local network, two devices on the same segment can communicate directly at the local network level, provided there is connectivity and policies allow it.
But sharing an IP subnet does not automatically guarantee communication: VLANs, wireless isolation, firewalls, and device configuration also play a role.
When the destination is outside the local subnet, the device usually sends traffic to a gateway, which routes it to another network.
Are subnet, VLAN, and private network the same thing?
No. A subnet belongs to IP address organization and routing. A VLAN allows separating network segments at the link layer on compatible equipment. A private network may refer to the use of non-public IP ranges or to a limited connectivity design.
In companies and data centers, subnets and VLANs are often used together, but they are not synonyms.
An example on a server
Imagine you have several services: a public proxy, a database, and internal tasks. You can organize them so that the proxy communicates with the necessary services and the database does not have to receive connections from the Internet.
The subnet helps define ranges and routes, but does not replace the firewall. Being within a private network also does not mean that all its users should access all applications.
Errors worth avoiding
Thinking that /24 means 24 devices. It means 24 bits of prefix, not 24 available addresses.
Copying any mask from a tutorial. The mask must match your actual network plan and the routes of the equipment.
Changing the server IP without reviewing the gateway. You could lose remote access if the new configuration stops reaching the router.
Assuming that a private IP implies security. It is just an addressing characteristic: permissions are configured separately.
Where does it appear in ArduMaker?
In ArduMaker, when working with VPS and containers connected through private networks, IP ranges and routes determine which equipment can find each other. Understanding a prefix like /24 is useful to avoid overlapping addresses and diagnose connectivity failures.
To expand on the topic, you can read how an IP address works and what a firewall is.
In one sentence: the mask defines which part of the IP corresponds to the network and which remains available within it. That small difference explains much of how home networks, companies, and servers are organized.
Sources: RFC 4632: CIDR addressing · What is a subnet, Cloudflare.