IP Addressing IPv4, IPv6, Classes, CIDR: Tutorial, Examples, FAQs & Interview Tips
IPv4 Addressing
An IPv4 address is a 32-bit number written in dotted decimal notation - four octets (8-bit groups) separated by dots. Each octet ranges from 0 to 255.
Example: 192.168.1.100 = 11000000.10101000.00000001.01100100 in binary
IPv4 Address Classes
| Class | First Octet Range | Default Subnet Mask | Networks | Hosts/Network | Use |
|---|
| A | 1 - 126 | 255.0.0.0 (/8) | 126 | 16,777,214 | Large organizations |
| B | 128 - 191 | 255.255.0.0 (/16) | 16,384 | 65,534 | Medium organizations |
| C | 192 - 223 | 255.255.255.0 (/24) | 2,097,152 | 254 | Small organizations |
| D | 224 - 239 | N/A | N/A | N/A | Multicast |
| E | 240 - 255 | N/A | N/A | N/A | Reserved/Experimental |
Special IP Addresses
| Address/Range | Purpose |
|---|
| 127.0.0.0 - 127.255.255.255 | Loopback (localhost) - 127.0.0.1 is most common |
| 10.0.0.0 - 10.255.255.255 | Private Class A (RFC 1918) |
| 172.16.0.0 - 172.31.255.255 | Private Class B (RFC 1918) |
| 192.168.0.0 - 192.168.255.255 | Private Class C (RFC 1918) |
| 169.254.0.0 - 169.254.255.255 | APIPA (Automatic Private IP Addressing) - when DHCP fails |
| 0.0.0.0 | Default route / unspecified address |
| 255.255.255.255 | Limited broadcast (all hosts on local network) |
Unicast, Multicast, and Broadcast
- Unicast: One-to-one communication. A packet is sent from one source to one specific destination. Most common type.
- Broadcast: One-to-all communication. A packet is sent to all devices on a network. Example: 192.168.1.255 (directed broadcast for 192.168.1.0/24).
- Multicast: One-to-many communication. A packet is sent to a group of interested receivers. Uses Class D addresses (224.0.0.0 - 239.255.255.255).
- Anycast: One-to-nearest communication. A packet is sent to the nearest node in a group. Used in IPv6 and CDNs.
IPv6 Addressing
IPv6 uses 128-bit addresses written as 8 groups of 4 hexadecimal digits separated by colons.
Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Abbreviation rules:
- Leading zeros in each group can be omitted:
0db8 -> db8
- One consecutive group of all-zero groups can be replaced with
::: 2001:db8::8a2e:370:7334
IPv4 vs IPv6
| Feature | IPv4 | IPv6 |
|---|
| Address Size | 32 bits | 128 bits |
| Address Format | Dotted decimal (192.168.1.1) | Hexadecimal colon notation |
| Total Addresses | ~4.3 billion | ~340 undecillion |
| Header Size | 20-60 bytes (variable) | 40 bytes (fixed) |
| NAT Required | Yes (address exhaustion) | No (enough addresses) |
| Security | Optional (IPSec) | Built-in (IPsec mandatory) |
| Broadcast | Yes | No (uses multicast/anycast) |
| Auto-configuration | DHCP | SLAAC (Stateless Address Autoconfiguration) |
IPv6 Address Types
| Type | Prefix | Purpose | Example |
|---|
| Global Unicast |
2000::/3 |
Routable Internet addresses |
2001:db8::1 |
| Link-Local |
fe80::/10 |
Communication on same link |
fe80::1 |
| Unique Local |
fc00::/7 |
Private addressing (like RFC 1918) |
fc00::1 |
| Multicast |
ff00::/8 |
One-to-many communication |
ff02::1 |
| Loopback |
::1 |
Local host (like 127.0.0.1) |
::1 |
| Unspecified |
:: |
Default route (like 0.0.0.0) |
:: |
Subnet Masking and CIDR
Subnet masks divide IP addresses into network and host portions. CIDR (Classless Inter-Domain Routing) replaces class-based addressing with flexible subnetting.
CIDR Notation
| CIDR | Subnet Mask | Network Bits | Host Bits | Total Hosts | Usable Hosts |
|---|
| /8 | 255.0.0.0 | 8 | 24 | 16,777,216 | 16,777,214 |
| /16 | 255.255.0.0 | 16 | 16 | 65,536 | 65,534 |
| /24 | 255.255.255.0 | 24 | 8 | 256 | 254 |
| /25 | 255.255.255.128 | 25 | 7 | 128 | 126 |
| /26 | 255.255.255.192 | 26 | 6 | 64 | 62 |
| /27 | 255.255.255.224 | 27 | 5 | 32 | 30 |
| /28 | 255.255.255.240 | 28 | 4 | 16 | 14 |
| /29 | 255.255.255.248 | 29 | 3 | 8 | 6 |
| /30 | 255.255.255.252 | 30 | 2 | 4 | 2 |
| /31 | 255.255.255.254 | 31 | 1 | 2 | 0 |
| /32 | 255.255.255.255 | 32 | 0 | 1 | 1 |
IP Address Assignment Methods
Dynamic Host Configuration Protocol (DHCP)
DHCP automatically assigns IP addresses to devices on a network. The DHCP process involves four steps:
- DHCPDISCOVER: Client broadcasts request for IP address
- DHCPOFFER: Server offers available IP address
- DHCPREQUEST: Client requests the offered address
- DHCPACK: Server acknowledges and assigns the address
Static vs Dynamic IP Assignment
| Method | Advantages | Disadvantages | Use Cases |
|---|
| Static IP |
Consistent address, no DHCP dependency, easy to remember |
Manual configuration, potential conflicts, less flexible |
Servers, printers, routers, infrastructure devices |
| Dynamic IP |
Automatic configuration, no conflicts, efficient address usage |
Address changes, requires DHCP server, less predictable |
Client computers, mobile devices, temporary connections |
Network Address Translation (NAT)
NAT allows multiple devices to share a single public IP address by translating private addresses to public ones. It was developed to address IPv4 address exhaustion.
NAT Types
- Static NAT: One-to-one mapping between private and public addresses
- Dynamic NAT: Many-to-many mapping from a pool of public addresses
- PAT (Port Address Translation): Many-to-one mapping using different ports (most common)
- NAT Overload: Another term for PAT, overloading a single public IP
NAT Benefits and Limitations
| Benefits | Limitations |
|---|
| Conserves IPv4 addresses |
Breaks end-to-end connectivity |
| Provides basic security (hides internal network) |
Problems with peer-to-peer applications |
| Allows network renumbering |
Complicates VoIP and video conferencing |
| Reduces need for public addresses |
Performance overhead |
IPv6 Address Configuration
Stateless Address Autoconfiguration (SLAAC)
SLAAC allows devices to automatically configure their own IPv6 addresses without a central server:
- Device creates link-local address (fe80::/10)
- Device discovers network prefix via Router Advertisement
- Device combines prefix with interface identifier
- Device performs Duplicate Address Detection (DAD)
Interface Identifier Generation
- EUI-64: Derived from MAC address (privacy concerns)
- Privacy Extensions: Random temporary addresses
- Stable Privacy: Random but stable addresses
- Manual: Manually configured interface ID
IP Address Planning and Best Practices
IPv4 Address Planning
- Use RFC 1918 private addresses for internal networks
- Reserve address ranges for different purposes (servers, printers, DHCP, etc.)
- Document address assignments in a central registry
- Plan for growth by allocating larger subnets initially
- Use consistent addressing schemes across locations
IPv6 Address Planning
- Assign /48 prefixes to sites for easy routing
- Use /64 subnets for LAN segments (standard practice)
- Implement IPv6 alongside IPv4 (dual-stack)
- Plan address hierarchy for efficient routing
- Consider privacy addressing for client devices
IP Address Troubleshooting
Common IP Address Issues
- IP Conflicts: Multiple devices with same IP address
- Incorrect Subnet Mask: Devices can't reach other networks
- Default Gateway Issues: No internet connectivity
- DNS Resolution Problems: Can't resolve domain names
- DHCP Failures: Devices can't get IP addresses
Troubleshooting Commands
| Command | Purpose | Example Usage |
|---|
| ipconfig / ifconfig |
Show IP configuration |
ipconfig /all (Windows) |
| ping |
Test connectivity |
ping 8.8.8.8 |
| tracert / traceroute |
Trace network path |
tracert google.com |
| nslookup |
DNS resolution test |
nslookup google.com |
| arp -a |
Show ARP table |
arp -a |
| netstat -rn |
Show routing table |
netstat -rn |
IP Security Considerations
Security Best Practices
- Use private addressing for internal networks (RFC 1918)
- Implement firewalls to control traffic between networks
- Disable unused services that might expose IP addresses
- Use IPsec for secure communication between sites
- Monitor for IP spoofing and unauthorized address usage
- Implement DHCP snooping to prevent rogue DHCP servers
Common IP-based Attacks
- IP Spoofing: Forging source IP addresses
- Smurf Attacks: ICMP echo requests to broadcast addresses
- SYN Floods: Overwhelming servers with connection requests
- DNS Amplification: Using DNS servers for DDoS attacks
- ARP Poisoning: Corrupting ARP tables for man-in-the-middle attacks
Future of IP Addressing
IPv6 Adoption Trends
- Mobile Networks: 4G/5G networks heavily use IPv6
- Cloud Services: Major cloud providers support IPv6
- Content Delivery: CDNs increasingly support IPv6
- Government Mandates: Some countries require IPv6 support
- IoT Devices: IPv6 essential for massive IoT deployments
Emerging Technologies
- IPv6-only Networks: Eliminating IPv4 entirely
- NAT64/DNS64: IPv6 to IPv4 translation mechanisms
- Segment Routing: Advanced routing with IPv6
- programmable addressing: SDN-based address management
- Zero Trust Networking: Identity-based access control
Related Networking Topics
