IP Addressing & Subnetting: A Comprehensive Guide
1) Introduction
IP addressing and subnetting are core concepts in networking. This guide provides a practical, in-depth walkthrough of IPv4/IPv6 addressing, subnet masks, CIDR, VLSM, route summarization, and plenty of worked examples with verification steps.
- Audience: Students, network technicians, and engineers preparing for certifications.
- Prerequisites: Basic binary/decimal familiarity and general network fundamentals.
- Outcomes: Design efficient subnets, calculate ranges, and validate configurations.
2) IPv4 Addressing Fundamentals
An IPv4 address is 32 bits, commonly written in dotted decimal (e.g., 192.168.10.42). Each octet ranges from 0–255.
- Binary structure: 32 bits split into 4 octets (8 bits each).
- Two portions: Network and Host.
- Mask defines how many bits belong to the network portion.
2.1 Classes (Legacy, for reference)
| Class | Range | Default Mask | Notes |
|---|---|---|---|
| A | 0.0.0.0 – 127.255.255.255 | 255.0.0.0 (/8) | Very large networks |
| B | 128.0.0.0 – 191.255.255.255 | 255.255.0.0 (/16) | Medium networks |
| C | 192.0.0.0 – 223.255.255.255 | 255.255.255.0 (/24) | Small networks |
| D | 224.0.0.0 – 239.255.255.255 | N/A | Multicast |
| E | 240.0.0.0 – 255.255.255.255 | N/A | Experimental |
2.2 Private vs Public IPv4
- 10.0.0.0/8
- 172.16.0.0/12
- 192.168.0.0/16
Private addresses are not routable on the public Internet and typically require NAT to reach external networks.
2.3 Special-use Addresses
- Loopback: 127.0.0.0/8 (commonly 127.0.0.1)
- Link-local: 169.254.0.0/16 (APIPA)
- Network/Direct broadcast (per subnet)
- RFC 1918 (private), RFC 5737 (documentation: 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24)
3) Subnet Masks and CIDR
The subnet mask determines how many bits identify the network. CIDR notation expresses this with a slash (e.g., /24).
3.1 Common Masks
| CIDR | Mask | # Hosts/Subnet | Increment |
|---|---|---|---|
| /24 | 255.255.255.0 | 254 | 1 in 4th octet |
| /25 | 255.255.255.128 | 126 | 128 in 4th octet |
| /26 | 255.255.255.192 | 62 | 64 in 4th octet |
| /27 | 255.255.255.224 | 30 | 32 in 4th octet |
| /28 | 255.255.255.240 | 14 | 16 in 4th octet |
| /29 | 255.255.255.248 | 6 | 8 in 4th octet |
| /30 | 255.255.255.252 | 2 | 4 in 4th octet |
- Block size: 64 (256 - 192)
- Subnets: 192.168.10.0, .64, .128, .192
- Pick subnet 192.168.10.128/26 → range .129 – .190, broadcast .191
3.2 Binary View
IP: 192.168.10.42 = 11000000.10101000.00001010.00101010
Mask: 255.255.255.192= 11111111.11111111.11111111.11000000 (/26)
Network:192.168.10.0 = 11000000.10101000.00001010.00000000
Host: = 00101010
4) VLSM (Variable Length Subnet Mask)
VLSM lets you allocate different mask lengths per subnet to fit host counts precisely.
- Sort by size: 60 → /26 (62 hosts), 30 → /27 (30 usable), 12 → /28 (14), 2 → /30 (2).
- Allocate sequentially:
- 10.10.10.0/26 → .1 – .62 (broadcast .63)
- 10.10.10.64/27 → .65 – .94 (broadcast .95)
- 10.10.10.96/28 → .97 – .110 (broadcast .111)
- 10.10.10.112/30 → .113 – .114 (broadcast .115)
- Remaining: 10.10.10.116 – 10.10.10.255 for future needs.
5) Route Summarization (Supernetting)
Summarization reduces routing table size by aggregating contiguous networks with common prefix bits.
Binary 3rd octet: 00001000, 00001001, 00001010, 00001011 → common prefix 000010
Summary: 172.16.8.0/22 (covers .8–.11)
6) Practical Subnetting Exercises
6.1 Exercise A
Network: 192.168.50.0/24. Create 5 subnets with ≥ 25 hosts each.
- Needed hosts per subnet: ≥25 → /27 (30 usable). Number of /27 subnets in /24: 8.
- Subnets: .0, .32, .64, .96, .128, .160, .192, .224
- Pick first five:
- .0/27 → .1 – .30 (broadcast .31)
- .32/27 → .33 – .62 (broadcast .63)
- .64/27 → .65 – .94 (broadcast .95)
- .96/27 → .97 – .126 (broadcast .127)
- .128/27 → .129 – .158 (broadcast .159)
6.2 Exercise B
From 10.0.0.0/8, carve out 16 equal subnets for WAN regions.
- 16 subnets → need 4 extra network bits → /12 (255.240.0.0)
- Regional blocks: 10.0.0.0/12, 10.16.0.0/12, ... 10.240.0.0/12 (increments of 16 in 2nd octet)
6.3 Exercise C
Given 172.20.100.0/23, find usable range for the second /24 inside.
- /23 spans 172.20.100.0 – 172.20.101.255
- /24 blocks inside: 172.20.100.0/24 and 172.20.101.0/24
- Second /24 usable: 172.20.101.1 – 172.20.101.254 (broadcast .255)
7) IPv6 Essentials
IPv6 uses 128-bit addresses in hexadecimal, separated by colons (e.g., 2001:0db8::1). It eliminates NAT in many designs and supports enormous addressing space.
7.1 Notation and Shortening
- Leading zeros can be omitted in each hextet.
- One sequence of consecutive all-zero hextets can be replaced with :: once.
- Example: 2001:0db8:0000:0000:0000:0000:0000:0001 → 2001:db8::1
7.2 Prefixes
- Typical LAN: /64
- Point-to-point: /127 (per RFC 6164) or /64 with SLAAC disabled
- Global Unicast: 2000::/3
- Link-local: fe80::/10
- Unique Local: fc00::/7 (commonly fd00::/8)
7.3 IPv6 Subnetting
Given 2001:db8:abcd::/48, create 4 /64 LANs.
- /48 → /64 needs 16 bits for subnet IDs.
- Allocate: 2001:db8:abcd:0000::/64, :0001::/64, :0002::/64, :0003::/64
8) Tools and Commands
8.1 Linux
# Show addresses and routes
ip addr
ip -4 route
ip -6 route
# Add address and default route
sudo ip addr add 192.168.10.10/24 dev eth0
sudo ip route add default via 192.168.10.1
8.2 Windows
# Show configuration ipconfig /all route print # Set IP (PowerShell) New-NetIPAddress -InterfaceAlias "Ethernet0" -IPAddress 192.168.10.10 -PrefixLength 24 -DefaultGateway 192.168.10.1 Set-DnsClientServerAddress -InterfaceAlias "Ethernet0" -ServerAddresses 1.1.1.1,8.8.8
.jpg)
0 Comments