255-255-255-0

That row of bits is the whole story. Twenty-four ones followed by eight zeros, which written in decimal is 255.255.255.0. The ones mark the part of an address that identifies the network. The zeros mark the part that identifies an individual device on it.

What the mask decides

Every device compares its own address against a neighbour’s using the mask. Matching network portions mean the two can talk directly over the local wire. Different network portions mean the traffic has to go through a router. That single comparison, done in hardware thousands of times a second, is why a mask matters at all.

With this mask, the first three numbers are fixed and the last one varies. On a 192.168.1.x network that gives you 256 combinations, minus two that are reserved, leaving 254 usable addresses. One of those goes to the router, which is why the count people quote is usually 253 for actual devices.

Reading a mask without the maths

A 255 in any position means that whole octet is locked to the network. A 0 means the octet is free for host numbering. So 255.255.255.0 locks three octets and frees one. Change the mask to 255.255.0.0 and you free two octets, giving 65,534 usable addresses instead of 254.

Anything in between is where the arithmetic gets awkward, because a partial octet like 192 or 224 splits the range into blocks. The calculator above handles those, but for home use you will almost never need them. Consumer routers ship with /24 and there is rarely a reason to change it.

Common masks and what they give you

The two addresses you cannot use

Where the gateway sits

Convention puts it at the first usable address, .1. Plenty of hardware follows that, which is why 192.168.1.1 shows up so often. Other vendors take the last usable address instead, giving you a .254 gateway. Neither is more correct; both are just habits.

Setting a mask by hand

• Use the same mask on every device in the subnet, without exception
• Match the gateway your router reports, not one from a guide
• Keep static addresses outside the DHCP pool so nothing gets handed out twice
• On a 192.168.x.x network, 255.255.255.0 is almost always the right answer

A mismatch is quietly destructive. One machine on /24 and another on /25 will each have a different idea of who counts as a local neighbour, producing a network where some devices see each other and some do not. It looks like a hardware fault and it is not. If you are pinning a device manually, the worked example in the static address guide shows the four values you need in the right order, and an address like 192.168.111.114 demonstrates how a lease lands somewhere unexpected inside a large pool.

Masks outside 192.168

The same rules apply to the other private blocks. A network built on 10.x can use /24 just as easily, which is common on hotspot equipment and business gear sitting behind a 10.10.0.1 gateway. The 10.0.0.0/8 block simply gives you room for far more subnets before you run out.

Where the mask is irrelevant

Loopback ignores all of this. Traffic sent to 127.0.0.1 never touches an interface, so no mask comparison happens. The full behaviour of that address is set out in the loopback explanation, and it is worth reading before assuming every numeric address obeys subnet rules.

FAQ