Intermediate Estimated time: 13 min Category: Networking Published

Reserved IP Ranges¶

Reserved IP addresses are address blocks that are not routed on the public Internet. They include private ranges, documentation ranges, loopback and other special uses defined by IANA and RFCs.

Private Ranges (RFC 1918)¶

Private Class A¶

Range	CIDR Prefix	Available Hosts	Use
10.0.0.0 - 10.255.255.255	10.0.0.0/8	16,777,214	Large corporate networks

Characteristics: - A single /8 block - Largest private address space - Common in large organizations - Supports VLSM for subnetting

Private Class B¶

Range	CIDR Prefix	Available Hosts	Use
172.16.0.0 - 172.31.255.255	172.16.0.0/12	1,048,574	Medium-sized networks

Characteristics: - 16 blocks of /16 each - Balance between size and flexibility - Common on university campuses - Easy to remember and manage

Private Class C¶

Range	CIDR Prefix	Available Hosts	Use
192.168.0.0 - 192.168.255.255	192.168.0.0/16	65,534	Small networks

Characteristics: - 256 blocks of /24 each - Easiest to manage - Common in homes and small offices - Native support in home routers

IANA Special Ranges¶

Loopback (RFC 5735)¶

Range	CIDR Prefix	Use
127.0.0.0 - 127.255.255.255	127.0.0.0/8	Local loopback interface

Details: - 127.0.0.1: Standard loopback - 127.0.0.0/8: The whole range is reserved - Not routable: Only reachable locally - Testing: Used for testing local services

Link-Local (RFC 3927)¶

Range	CIDR Prefix	Use
169.254.0.0 - 169.254.255.255	169.254.0.0/16	Automatic autoconfiguration

Details: - APIPA: Automatic Private IP Addressing - Windows: Assigned when DHCP fails - Zeroconf: Used by Bonjour, Avahi - Not routable: Local link only

TEST-NET (RFC 5737)¶

Range	CIDR Prefix	Use
192.0.2.0 - 192.0.2.255	192.0.2.0/24	Documentation and examples
198.51.100.0 - 198.51.100.255	198.51.100.0/24	Documentation and examples
203.0.113.0 - 203.0.113.255	203.0.113.0/24	Documentation and examples

Details: - RFC 5737: Ranges for documentation - Do not use in production: Examples only - Books/Tutorials: Common in technical documentation

CGNAT (Carrier-Grade NAT) Ranges¶

RFC 6598 (CGNAT)¶

Range	CIDR Prefix	Use
100.64.0.0 - 100.127.255.255	100.64.0.0/10	Shared Address Space

Details: - CGNAT: Carrier-Grade Network Address Translation - ISPs: Used by Internet service providers - RFC 6598: Standard for shared address space - Not routable: Only within carrier networks

CGNAT Usage Examples¶

ISP Network:
- Public IPs: 203.0.113.0/24 (public range)
- CGNAT Pool: 100.64.0.0/16 (for customers)
- Customer 1: 100.64.1.1 (NAT to 203.0.113.10)
- Customer 2: 100.64.1.2 (NAT to 203.0.113.10)

IANA Reserved Ranges¶

IPv4 Special-Purpose Address Registry¶

Range	CIDR Prefix	RFC	Purpose
0.0.0.0/8	0.0.0.0/8	RFC 1122	"This" network
192.0.0.0/24	192.0.0.0/24	RFC 5736	IETF Protocol Assignments
192.0.2.0/24	192.0.2.0/24	RFC 5737	TEST-NET-1
198.51.100.0/24	198.51.100.0/24	RFC 5737	TEST-NET-2
203.0.113.0/24	203.0.113.0/24	RFC 5737	TEST-NET-3
240.0.0.0/4	240.0.0.0/4	RFC 1112	Class E (experimental)

Ranges by Continent (RIRs)¶

Private ranges do not depend on the RIR

The five regional registries (ARIN, RIPE NCC, APNIC, LACNIC and AFRINIC) hand out public address space. The RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not assigned by any RIR: they are the same worldwide and anyone can use them inside their own network without requesting them.

IPv6 Reserved Ranges¶

Unique Local Addresses (ULA)¶

Range	Prefix	RFC	Use
fc00::/7	fc00::/7	RFC 4193	Unique local addresses

Details: - fc00::/8: Assigned by LIRs - fd00::/8: Locally generated - Not routable: Within the site only

Link-Local Unicast¶

Range	Prefix	RFC	Use
fe80::/10	fe80::/10	RFC 4291	Link-local

Details: - fe80::/64: Per interface - Autoconfiguration: SLAAC - Not routable: Local link only

Multicast¶

Range	Prefix	RFC	Use
ff00::/8	ff00::/8	RFC 4291	Multicast addresses

Important groups: - ff02::1: All nodes - ff02::2: All routers - ff05::2: All OSPF routers

Device Configuration¶

Cisco Router (IOS)¶

! Configure interface with a private IP
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 no shutdown

! Configure NAT for Internet access
ip nat inside source list 1 interface GigabitEthernet0/1 overload
access-list 1 permit 192.168.1.0 0.0.0.255

! Configure DHCP for clients
ip dhcp pool LAN
 network 192.168.1.0 255.255.255.0
 default-router 192.168.1.1
 dns-server 8.8.8.8

Linux (netplan)¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
      gateway4: 192.168.1.1
      nameservers:
        addresses:
          - 8.8.8.8
          - 1.1.1.1

Windows (PowerShell)¶

# Configure a private IP
New-NetIPAddress -InterfaceAlias "Ethernet" -IPAddress 192.168.1.100 -PrefixLength 24 -DefaultGateway 192.168.1.1

# Configure DNS
Set-DnsClientServerAddress -InterfaceAlias "Ethernet" -ServerAddresses ("8.8.8.8","1.1.1.1")

Security Considerations¶

Risks of Private Ranges¶

IP conflicts: Multiple networks using the same range
Accidental routing: Insufficient filtering
Internal attacks: Without proper segmentation

Best Practices¶

Network Segmentation¶

# Linux - Create VLANs
ip link add link eth0 name eth0.10 type vlan id 10
ip addr add 192.168.10.1/24 dev eth0.10

# Firewall rules
iptables -A FORWARD -s 192.168.1.0/24 -d 192.168.2.0/24 -j DROP

Range Monitoring¶

#!/bin/bash
# Scan the private network for devices

NETWORK="192.168.1.0/24"

echo "Scanning $NETWORK..."
nmap -sn $NETWORK | grep "Nmap scan report" | awk '{print $5}' > hosts.txt

echo "Hosts found:"
cat hosts.txt

VPN and Private Ranges¶

OpenVPN with private ranges¶

server 10.8.0.0 255.255.255.0
push "route 192.168.1.0 255.255.255.0"

WireGuard¶

[Interface]
Address = 10.0.0.1/24
PrivateKey = ...

[Peer]
AllowedIPs = 10.0.0.2/32, 192.168.1.0/24

Troubleshooting¶

Common Problems¶

1. IP Conflict¶

# Check IPs in use
arp -a
nmap -sn 192.168.1.0/24

2. No connectivity¶

# Check IP configuration
ip addr show
ip route show

# Test connectivity
ping 192.168.1.1
traceroute 8.8.8.8

3. DNS not working¶

# Check DNS servers
cat /etc/resolv.conf
nslookup google.com

Diagnostic Tools¶

# View the ARP table
arp -n

# View routes
ip route

# View interfaces
ip link show

# Connectivity test
mtr 8.8.8.8

# View network processes
netstat -tlnp
ss -tlnp