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Understanding IP Address Types: Public vs Private vs Reserved

by ServerLookup Team
NetworkingIP AddressesTutorial

Understanding IP Address Types: Public vs Private vs Reserved

IP addresses are the fundamental addressing system of the internet, but not all IP addresses serve the same purpose. Understanding the different types of IP addresses is essential for network configuration, security, and troubleshooting.

What is an IP Address?

An IP (Internet Protocol) address is a unique numerical identifier assigned to each device connected to a network. Think of it as a postal address for your computer or server.

There are two versions currently in use:

  • IPv4: 32-bit addresses (e.g., 192.168.1.1)
  • IPv6: 128-bit addresses (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334)

Public IP Addresses

What Are Public IPs?

Public IP addresses are globally unique addresses assigned by Internet Assigned Numbers Authority (IANA) and regional internet registries. These addresses are routable on the internet.

Characteristics

  • Unique: No two devices on the public internet can have the same public IP
  • Routable: Can be reached from anywhere on the internet
  • Assigned: Allocated by ISPs or hosting providers
  • Limited: IPv4 addresses are scarce and expensive

Examples

8.8.8.8 (Google DNS)
1.1.1.1 (Cloudflare DNS)
142.250.185.46 (Google servers)

Use Cases

  • Web servers
  • Mail servers
  • Public-facing services
  • Remote access endpoints
  • CDN nodes

Private IP Addresses

What Are Private IPs?

Private IP addresses are reserved for use within private networks and are not routable on the public internet. They can be reused across different private networks.

IPv4 Private Ranges

Defined by RFC 1918:

10.0.0.0 to 10.255.255.255 (10.0.0.0/8)
- 16,777,216 addresses
- Commonly used in large enterprises

172.16.0.0 to 172.31.255.255 (172.16.0.0/12)
- 1,048,576 addresses
- Often used in medium-sized networks

192.168.0.0 to 192.168.255.255 (192.168.0.0/16)
- 65,536 addresses
- Most common in home and small office networks

IPv6 Private Ranges

fc00::/7 (Unique Local Addresses - ULA)
fe80::/10 (Link-Local Addresses)

Characteristics

  • Reusable: Same IP can be used in different private networks
  • Non-routable: Cannot be accessed directly from the internet
  • NAT Required: Need Network Address Translation to access the internet
  • Free: No cost to use these addresses

Use Cases

  • Home networks
  • Office networks
  • Internal servers
  • IoT devices
  • Virtual machines

Reserved and Special IP Addresses

Loopback Addresses

IPv4: 127.0.0.0/8 (typically 127.0.0.1) IPv6: ::1

Used to test network software by routing packets back to the same device.

# Test local network stack
ping 127.0.0.1

Link-Local Addresses

IPv4: 169.254.0.0/16 IPv6: fe80::/10

Automatically assigned when DHCP is unavailable. Only valid on the local network segment.

Multicast Addresses

IPv4: 224.0.0.0/4 IPv6: ff00::/8

Used for one-to-many communication. Examples:

  • 224.0.0.1: All hosts on local network
  • 224.0.0.2: All routers on local network

Broadcast Address

IPv4 Only: Last address in subnet (e.g., 192.168.1.255 in 192.168.1.0/24)

Sends packets to all devices in the subnet.

Documentation and Examples

Reserved for documentation and examples (never used in real networks):

IPv4:

  • 192.0.2.0/24 (TEST-NET-1)
  • 198.51.100.0/24 (TEST-NET-2)
  • 203.0.113.0/24 (TEST-NET-3)

IPv6:

  • 2001:db8::/32

Network Address Translation (NAT)

How NAT Works

NAT allows multiple devices with private IP addresses to share a single public IP address:

  1. Device with private IP sends packet to internet
  2. Router replaces source IP with its public IP
  3. Router tracks the connection
  4. Return packets are routed back to the correct private IP

Types of NAT

Static NAT: One-to-one mapping between private and public IP

Dynamic NAT: Pool of public IPs shared by private IPs

PAT (Port Address Translation): Most common; uses ports to multiplex connections

NAT Benefits

  • Conserves public IP addresses
  • Adds security layer (hides internal network)
  • Simplifies network management

NAT Drawbacks

  • Complicates peer-to-peer connections
  • Breaks end-to-end connectivity
  • Can cause issues with certain protocols
  • Adds latency and complexity

Checking Your IP Address

Public IP

Check your public-facing IP address:

# Using curl
curl ifconfig.me
curl ipinfo.io/ip

# Using dig
dig +short myip.opendns.com @resolver1.opendns.com

Or use tools like ServerLookup.io

Private IP

Check your local network IP:

# Windows
ipconfig

# Linux/Mac
ifconfig
ip addr show

Subnet Masks and CIDR Notation

Subnet Masks

Subnet masks define the network and host portions of an IP address:

255.255.255.0 = /24 (256 addresses, 254 usable)
255.255.0.0 = /16 (65,536 addresses)
255.0.0.0 = /8 (16,777,216 addresses)

CIDR Notation

Classless Inter-Domain Routing (CIDR) provides a more flexible way to define IP ranges:

192.168.1.0/24 = 192.168.1.0 to 192.168.1.255
10.0.0.0/8 = 10.0.0.0 to 10.255.255.255
172.16.0.0/12 = 172.16.0.0 to 172.31.255.255

IPv4 vs IPv6

IPv4 Exhaustion

  • Only ~4.3 billion addresses
  • Exhausted in most regions
  • Expensive to acquire
  • Requires NAT for most networks

IPv6 Advantages

  • 340 undecillion addresses (essentially unlimited)
  • No need for NAT
  • Better security with IPsec built-in
  • Simplified header format
  • Improved routing efficiency

IPv6 Adoption

Despite advantages, IPv6 adoption is still growing:

  • Many networks run dual-stack (IPv4 + IPv6)
  • Legacy systems require IPv4
  • Transition takes time

Best Practices

Network Planning

  1. Use private IPs internally: Save public IPs for external services
  2. Plan subnets carefully: Leave room for growth
  3. Document IP allocation: Maintain IP address management (IPAM)
  4. Implement proper security: Don't rely solely on private IPs for security

Security Considerations

  1. Firewall private ranges: Block private IPs at network edge
  2. Validate user input: Don't trust client-provided IPs
  3. Use VPNs: For secure remote access to private networks
  4. Monitor unusual traffic: Watch for IP spoofing attempts

Troubleshooting

  1. Check IP configuration: Ensure correct subnet and gateway
  2. Verify NAT rules: Ensure port forwarding is correct
  3. Test connectivity: Use ping, traceroute, and nslookup
  4. Check DNS: IP address is correct but DNS might be wrong

Common Scenarios

Home Network

  • ISP assigns one public IP to your router
  • Router uses NAT to share it with all devices
  • Devices get private IPs from 192.168.x.x range
  • DHCP assigns IPs automatically

Enterprise Network

  • Multiple public IPs for servers
  • Large private IP space (10.x.x.x)
  • VLANs for network segmentation
  • Complex NAT and firewall rules

Cloud Infrastructure

  • Public IPs for load balancers
  • Private IPs for backend servers
  • VPC (Virtual Private Cloud) networking
  • Security groups control access

Conclusion

Understanding IP address types is fundamental to networking. Whether you're setting up a home network, configuring a server, or troubleshooting connectivity issues, knowing when to use public, private, or reserved IP addresses is essential.

Key takeaways:

  • Public IPs are for internet-facing services
  • Private IPs are for internal networks
  • NAT bridges private and public networks
  • IPv6 will eventually replace IPv4
  • Reserved ranges serve special purposes

Tools like ServerLookup.io can help you inspect IP addresses, check their type, and gather information about their ownership and location. Understanding these concepts will make you a more effective network administrator or developer.