~3 min read • Updated Aug 3, 2025
1. What is Subnetting?
Subnetting is the process of dividing a large IP network into smaller subnetworks to reduce broadcast domains, simplify management, and improve performance and security.
2. Subnet Design Steps
- Determine the number of required physical or WAN segments
- Calculate how many hosts are needed per subnet
- Select the proper subnet mask and define valid host address ranges
3. Powers of 2 for Subnetting
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
2^5 = 32
2^6 = 64
2^7 = 128
2^8 = 256
2^9 = 512
2^10 = 1024
4. Default Masks for IP Classes
- Class A: 255.0.0.0 (/8)
- Class B: 255.255.0.0 (/16)
- Class C: 255.255.255.0 (/24)
5. CIDR and Subnet Masks
| Subnet Mask | CIDR |
|---|---|
| 255.255.255.128 | /25 |
| 255.255.255.192 | /26 |
| 255.255.255.224 | /27 |
| 255.255.255.240 | /28 |
| 255.255.255.248 | /29 |
| 255.255.255.252 | /30 |
6. Mental Subnetting (Class C)
Block size = 256 - subnet mask
Subnets = 2^x (x = number of 1 bits)
Hosts per subnet = 2^y - 2 (y = number of 0 bits)
7. Example: 192.168.10.0/25
Subnets: 2
Hosts per subnet: 126
Valid subnets: 0 and 128
Broadcast addresses: 127 and 255
Host ranges:
Subnet 0: 192.168.10.1 to 192.168.10.126
Subnet 128: 192.168.10.129 to 192.168.10.254
8. IP Troubleshooting Steps
ping 127.0.0.1 → test TCP/IP stack
ping local IP → test network card
ping default gateway → test LAN connection
ping remote server → test WAN or DNS routing
9. Key CLI Tools
- ping
- tracert
- arp -a
- ipconfig /all
10. Common IP Addressing Errors
Error: Gateway address is set as broadcast address
Fix: Use a valid host IP from the subnet
Error: Host IP equals broadcast address
Fix: Pick an address between network and broadcast
11. Static IP Assignment Example
Network: 192.168.20.24/29
Mask: 255.255.255.248
Router: 192.168.20.25
Server: 192.168.20.30
Gateway: 192.168.20.25
12. What is NAT?
Network Address Translation maps private IPs to public IPs, allowing secure internet access and IPv4 address conservation.
13. NAT Pros and Cons
| Pros | Cons |
|---|---|
| Preserves public IPs | Harder to trace hosts |
| Provides flexibility | Some apps may fail with NAT |
| Improves security | Adds translation latency |
14. NAT Types
- SNAT: Static mapping of private to public IP
- DNAT: Dynamic mapping to available public IPs
- PAT: Overloading many private IPs using ports
15. NAT Terminology
- Inside Local: private IP before translation
- Inside Global: translated public IP
- Outside Local: destination IP before translation
- Outside Global: final destination public IP
16. PAT Example
10.1.1.1:5000 → 203.0.113.1:5000
10.1.1.2:6000 → 203.0.113.1:6000
Response packets are mapped back via NAT table
17. Port Forwarding in NAT
203.0.113.1:80 → 192.168.1.10:80 (Internal Web Server)
Written & researched by Dr. Shahin Siami