Hosts file

3.4 Given a scenario, implement IPv4 and IPv6 network services

Name Resolution

📘CompTIA Network+ (N10-009)

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The Hosts file is a simple, local way for a computer to map hostnames (like server1.example.com) to IP addresses (like 192.168.1.10). It works before a DNS (Domain Name System) lookup is even performed.

Think of it as a local mini-DNS system that lives on your computer.

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Where the Hosts File Lives

• Windows: C:\Windows\System32\drivers\etc\hosts
• Linux / macOS: /etc/hosts

Only users with administrative/root privileges can modify the Hosts file.

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Purpose of the Hosts File

1. Name resolution without DNS:
   If DNS isn’t available or you want to override it, the Hosts file can directly resolve hostnames to IP addresses.
2. Quick testing or troubleshooting:
   • For example, if you are testing a new web server before updating DNS, you can map the hostname to its IP in the Hosts file.
3. Blocking unwanted domains:
   • Some use it to block ads or malicious sites by pointing a hostname to 127.0.0.1 (the local computer).

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How the Hosts File Works

When your computer tries to connect to a hostname:

1. Check the Hosts file first.
   • If the hostname exists there, the computer uses the IP in the file.
2. If not found, query DNS.
   • This is why the Hosts file can “override” DNS resolution.

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Format of the Hosts File

Each line in the Hosts file typically has this structure:

<IP address> <hostname> [alias]

• IP address: The IP to map (IPv4 or IPv6).
• Hostname: The fully qualified domain name (FQDN).
• Alias (optional): A shorter or alternative name.

Example (Windows/Linux):

192.168.1.10   server1.example.com  server1
127.0.0.1      localhost
::1            localhost

Explanation:

• 192.168.1.10 maps to server1.example.com and also server1 as a shortcut.
• 127.0.0.1 is the IPv4 loopback address (the local computer itself).
• ::1 is the IPv6 loopback address.

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IPv4 and IPv6 Support

• The Hosts file can handle IPv4 (192.168.1.10) and IPv6 (2001:db8::1).
• For IPv6, the format is the same: just use the IPv6 address instead of IPv4.

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Benefits of Using the Hosts File

• Fast resolution because it doesn’t require querying an external DNS server.
• Control over hostname mapping for testing or troubleshooting.
• Overrides DNS when needed.

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Limitations of the Hosts File

• Manual maintenance: Each computer needs its own updated file.
• Not scalable: For large networks, managing hosts files on many computers is impractical.
• Can be overridden: Some malware may change the Hosts file to redirect traffic.

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Exam Tips for CompTIA Network+

• Know that Hosts file is a local method of name resolution.
• It’s checked before DNS, so it can override DNS entries.
• Remember the file location for Windows (C:\Windows\System32\drivers\etc\hosts) and Linux/macOS (/etc/hosts).
• Recognize the format: <IP address> <hostname> [alias].
• Understand that it supports both IPv4 and IPv6.

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✅ Summary for your students:
The Hosts file is like a personal, local mini-DNS for your computer. It’s useful for testing, troubleshooting, or overriding DNS entries. However, it’s not practical for large networks, and it must be maintained manually.