Hosts file: definition, location, and syntax (2026 guide)

The hosts file is one of the oldest and most powerful tools in network computing. Present on all modern operating systems, it allows you to control DNS resolution locally on your machine. In this comprehensive guide, we'll explore everything you need to know about the hosts file in 2026.

What is the hosts file?

The hosts file is a plain text file that maps domain names (like google.com or mysite.local) to IP addresses. It's the ancestor of the DNS system as we know it today.

A bit of history

Before the invention of DNS in 1983, all computers connected to ARPANET (the precursor to the Internet) used a centralized hosts file to resolve names to IP addresses. This file was manually maintained by the Stanford Research Institute and distributed to all hosts on the network.

Today, although DNS has replaced this centralized system, the hosts file remains present on all operating systems to allow local overrides of DNS resolution.

How does DNS resolution work?

When you type a web address in your browser, here's what happens:

• Your system first checks the local hosts file
• If the domain isn't found, it queries the local DNS cache
• Then it contacts the configured DNS server (usually your ISP's or a public DNS like 8.8.8.8)
• The DNS server returns the corresponding IP address

The hosts file has absolute priority over all other DNS resolution mechanisms. This is what makes it so powerful for local development and testing.

Hosts file location by system

The hosts file location varies depending on your operating system:

On macOS

/etc/hosts

To edit it, you'll need administrator rights:

sudo nano /etc/hosts

On Windows

C:\Windows\System32\drivers\etc\hosts

Open Notepad as administrator, then navigate to this file.

On Linux

/etc/hosts

Like on macOS, use sudo to edit it:

sudo nano /etc/hosts

Tip: On all these systems, the hosts file is write-protected for security reasons. You must always have administrator rights to modify it.

Hosts file syntax and format

The hosts file format is simple but strict. Each line follows this structure:

IP_address    domain_name    [aliases...]

Essential syntax rules

• One entry per line: each IP/domain association occupies its own line
• Separator: use spaces or tabs between the IP and domain
• Comments: lines starting with # are ignored
• Aliases: you can add multiple domains on the same line

Concrete examples

# Standard localhost
127.0.0.1       localhost
::1             localhost

# Local development projects
127.0.0.1       myproject.local
127.0.0.1       api.myproject.local
127.0.0.1       admin.myproject.local

# Block a site
0.0.0.0         distracting-site.com
0.0.0.0         www.distracting-site.com

# Redirect to a staging server
192.168.1.100   staging.mycompany.com

IPv4 vs IPv6

The hosts file supports both IP address formats:

• IPv4: classic format like 127.0.0.1 or 192.168.1.1
• IPv6: modern format like ::1 (localhost) or 2001:db8::1

For maximum compatibility, often declare both:

127.0.0.1       mysite.local
::1             mysite.local

Common hosts file use cases

Local web development

This is the most common use of the hosts file for developers. Instead of accessing your projects via localhost:3000 or 127.0.0.1, you can create more realistic local domains:

127.0.0.1       myapp.local
127.0.0.1       api.myapp.local
127.0.0.1       cdn.myapp.local

Advantages:

• More readable and memorable URLs
• Simulation of multi-domain environments
• Cross-domain cookie testing
• More realistic CORS configuration

Testing before DNS migration

When migrating a site to a new server, you can test by temporarily pointing the domain to the new IP:

# Test the new server before changing official DNS
203.0.113.50    www.mysite.com
203.0.113.50    mysite.com

Only you will see the new server, while the rest of the world continues to access the old one.

Blocking unwanted sites

By redirecting domains to a non-existent address, you can block access to certain sites:

# Block distractions
0.0.0.0         facebook.com
0.0.0.0         www.facebook.com
0.0.0.0         twitter.com

# Block advertising
0.0.0.0         ads.doubleclick.net

For more on blocking, check our complete guide to blocking ads with the hosts file.

Multi-environment management

Freelancers and teams managing multiple projects can organize their entries by client:

# Client A
127.0.0.1       client-a.local
127.0.0.1       api.client-a.local

# Client B
127.0.0.1       client-b.local

Common errors and solutions

Error: "Permission denied"

Symptom: Unable to save the hosts file.

Cause: The file is write-protected.

Solution: Use sudo on macOS/Linux, or open your editor as administrator on Windows. For a detailed Mac guide, check our article How to edit the hosts file on Mac.

Error: Changes not taking effect

Symptom: You modified the file, but the browser still shows the old IP.

Cause: DNS cache wasn't flushed.

Solution: Flush your system's DNS cache.

On macOS:

sudo dscacheutil -flushcache
sudo killall -HUP mDNSResponder

On Windows:

ipconfig /flushdns

Error: Invalid syntax

Symptom: Some entries work, others don't.

Solutions:

• Rewrite entries manually (avoid copy-paste)
• Check for invisible spaces
• Follow the format: IP then domain

Error: .local domains slow on macOS

Cause: macOS uses Bonjour/mDNS for .local domains.

Solution: Use .test or .localhost instead of .local.

Tools to simplify management

Manual hosts file management via terminal works but has drawbacks:

• Risk of syntax errors
• No automatic validation
• Hard to find entries in large files

Applications like Locahl offer a modern alternative with a visual interface, automatic validation, and one-click DNS flush.

Best practices

Organize your entries

Use comments to structure your file:

# ===================
# LOCALHOST
# ===================
127.0.0.1       localhost

# ===================
# PERSONAL PROJECTS
# ===================
127.0.0.1       blog.local
127.0.0.1       portfolio.local

Back up regularly

Before any major modification:

sudo cp /etc/hosts /etc/hosts.backup

Use appropriate TLDs

• .test: reserved by IETF, recommended
• .localhost: always local
• .local: can conflict with Bonjour on Mac
• .dev: real domain, requires HTTPS

Conclusion

The hosts file remains a fundamental tool for every developer in 2026. Its simplicity and effectiveness make it a valuable ally for local development, migration testing, and site blocking. Mastering the hosts file will save you precious time every day.