IP address, subnet mask, gateway: the three settings to understand

Every machine connected to an IPv4 network has at least three settings: an IP address, a subnet mask and a default gateway. Together, they answer two questions: "who am I on this network?" and "which way do I go to reach anywhere else?".

A fourth setting, the DNS server, turns names (example.com) into addresses. It is covered at the end of this guide.

1. The IP address: the machine's identity

An IPv4 address is written as four numbers from 0 to 255 separated by dots: 192.168.1.42. Each number takes one byte, or 8 bits: an address is therefore 32 bits long.

On a given network, two machines cannot share the same address. If that happens, one of them (or both) loses its connection on and off: that is an address conflict.

An IP address is read in two parts:

• the network part, shared by every machine on the same local network;
• the host part, specific to each machine.

Where is the boundary between the two? That is the mask's job.

2. The subnet mask: the boundary

The mask says how many bits, counting from the left, belong to the network part. The most common mask at home is 255.255.255.0:

The first three octets identify the network (192.168.1), the last one identifies the machine (42). Every machine in 192.168.1.x is therefore on the same local network.

In binary, 255 is written 11111111: the mask 255.255.255.0 lines up 24 bits set to 1, then 8 bits set to 0. Hence its short form, CIDR notation: 192.168.1.42/24. Both forms say exactly the same thing.

Addresses you cannot give to a machine

In a /24 network, two addresses are reserved:

3. The gateway: the way out

A machine can only talk directly to machines on its own network. For everything else (the internet, another company network), it hands its packets to a router: the default gateway. At home, it is your internet box or router, often at 192.168.1.1 or 192.168.1.254.

4. How a machine decides where to send a packet

This is where the three settings work together. Before sending a packet, the machine applies its mask to its own address and to the destination address (a bitwise logical AND). If both results are identical, the destination is on the same network.

Take the machine 192.168.1.42/24, gateway 192.168.1.1:

To deliver a packet on the local network, the machine still needs the physical address (MAC address) of the recipient or of the gateway: it asks for it with the ARP protocol ("who has address 192.168.1.50?"), and keeps the answer in memory for a few minutes.

5. Viewing your settings

Windows

In a terminal (PowerShell or Command Prompt):

ipconfig

Excerpt of the output:

Ethernet adapter Ethernet:
   IPv4 Address. . . . . . . . . . . : 192.168.1.42
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 192.168.1.1

ipconfig /all adds the MAC address, the DNS servers and the DHCP status. In PowerShell, Get-NetIPConfiguration shows the same information adapter by adapter.

Linux

ip -4 addr show
ip route

Excerpts:

inet 192.168.1.42/24 brd 192.168.1.255 scope global dynamic enp3s0
default via 192.168.1.1 dev enp3s0 proto dhcp

The first line gives the address and mask in CIDR notation, plus the broadcast address (brd). The default via line gives the gateway. DNS servers are shown with resolvectl status on distributions that use systemd-resolved (Ubuntu in particular).

macOS

networksetup -getinfo Wi-Fi

Result:

DHCP Configuration
IP address: 192.168.1.42
Subnet mask: 255.255.255.0
Router: 192.168.1.1

Replace Wi-Fi with Ethernet for a wired connection (networksetup -listallnetworkservices lists the exact names). DNS servers are shown with scutil --dns.

6. Automatic (DHCP) or static?

Most of the time, these settings are handed out automatically by a DHCP server built into the box. The machine asks for an address at startup and receives the address, mask, gateway and DNS servers for a limited time (the lease).

A server, a NAS or a printer, on the other hand, must always keep the same address, so you can find it. Two methods:

7. The fourth setting: DNS

Machines talk to each other with IP addresses, but we use names. The DNS server does the translation: example.com → an IP address. At home, the box often acts as a DNS relay; you can also set a public resolver or a local DNS server.

Typical symptom of a DNS problem: ping 1.1.1.1 answers, but no website opens by its name. The way to check it is detailed in network troubleshooting: the essential commands.

8. The most common mistakes

Key takeaways

• IP address: the machine's identity, unique on the network.
• Mask: separates the network part from the host part; 255.255.255.0 is also written /24.
• Gateway: the router that takes packets out of the local network; it is always on the same network as the machine.
• DNS: turns names into addresses.
• For a server, prefer a DHCP reservation to an address typed by hand.