What are private IP addresses?

Private IP addresses are special IPv4 address ranges intended for use only within local networks (LAN) and not routed on the internet.

Short wording for an interview:

Private IP addresses are addresses reserved for internal networks, which are not used on the global Internet and require NAT to reach the outside world.

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#Why private addresses are needed

• So that any number of devices inside an office, home, or company can have unique addresses.
• To save public IPv4 addresses, which are scarce.
• To ensure security: internal nodes are not visible from the internet.

Main idea:

The internal network uses a private IP, and it reaches the Internet through NAT.

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#Ranges of private IPv4 addresses

RFC 1918 defines three ranges:

#1. 10.0.0.0/8

From 10.0.0.0 to 10.255.255.255 Pros: a huge range (16 million hosts). Used in large companies, VPNs, and data centers.

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#2. 172.16.0.0/12

From 172.16.0.0 to 172.31.255.255 Used less often, but also common in corporate networks.

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#3. 192.168.0.0/16

From 192.168.0.0 to 192.168.255.255 The most common home and office subnets:

• 192.168.0.0/24
• 192.168.1.0/24
• 192.168.100.0/24

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#Important properties of private addresses

#1. Not routed on the internet

Routers filter private ranges: packets do not pass through the WAN.

#2. Require NAT to reach the internet

Private IP → NAT/PAT → Public IP This is done by a home router or a corporate firewall.

#3. Can be reused many times

The same addresses can repeat across millions of different networks.

#4. Not globally unique

The address 192.168.1.10 can exist at your home, at the office, and at your neighbor's - that is normal.

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#Usage examples

• Home network:
  • 192.168.0.10: laptop
  • 192.168.0.1: router
• Corporate network:
  • 10.0.12.15: workstation
  • 10.1.0.5: file server
• ISP / VPN:
  • 172.16.x.x: different company branches

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#Why private IP is secure

A device with a private IP:

• is not visible from the internet
• is not directly accessible by IP
• requires port forwarding
• is protected by NAT + a firewall

This reduces the likelihood of external attacks.

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#Private vs public IP

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#Common confusion: 192.0.2.x, 198.18.x.x are not private

There are other special ranges too:

• 192.0.2.0/24: documentation
• 198.18.0.0/15: testing But they are not private: do not confuse them with RFC 1918.