PHASE 05

IP Addressing & Subnetting Mastery

Classful vs Classless IPv4, binary bit-borrowing, Subnet Mask ANDing, FLSM, VLSM, CIDR prefix rules, and step-by-step mathematical calculations.

1. Interactive Binary Bit-Borrowing Subnet Visualizer

2. Classful IP Addressing vs. Classless Schemes

Legacy Inefficient

Classful IP Addressing

Historic rigid allocation based on the leading bits of the first octet:

Class A (1-127): /8 Mask (255.0.0.0) $\rightarrow$ 16,777,214 hosts
Class B (128-191): /16 Mask (255.255.0.0) $\rightarrow$ 65,534 hosts
Class C (192-223): /24 Mask (255.255.255.0) $\rightarrow$ 254 hosts
Class D (224-239): Multicast traffic

❌ Wasted millions of IP addresses globally.

Modern Enterprise Standard

Classless Addressing (CIDR, FLSM, VLSM)

Arbitrary network prefix length specified via slash notation (e.g. `/26`):

CIDR: Classless Inter-Domain Routing
FLSM: Fixed Length Subnet Masking (equal-sized subnets)
VLSM: Variable Length Subnet Masking (custom subnet sizes matched to exact department user numbers)

✓ Prevents address exhaustion and enables route summarization.

3. CIDR Prefix Reference Cheat Sheet & Bit Borrowing Rules

CIDR Subnet Mask & Usable Host Reference Table


CIDR    Subnet Mask          Borrowed Bits   Host Bits (h)   Total Hosts   Usable Hosts ($2^h - 2$)
/24     255.255.255.0        0               8               256           254
/25     255.255.255.128      1               7               128           126
/26     255.255.255.192      2               6               64            62
/27     255.255.255.224      3               5               32            30
/28     255.255.255.240      4               4               16            14
/29     255.255.255.248      5               3               8             6
/30     255.255.255.252      6               2               4             2  (Point-to-Point WAN)
/31     255.255.255.254      7               1               2             2  (Special Router CIDR*)

💡 Special Lecturer Rule: The /31 Router Link Exception

Notice for /31 CIDR prefixes (connecting 2 point-to-point routers): We do NOT subtract 2 because routers do not broadcast. Both available addresses are assigned directly to the router interfaces!

4. Step-by-Step VLSM Calculation Workflow

Example Problem: Allocate Subnets for Network 10.1.1.0/24

Department requirements: Admin (150 hosts), Marketing (60 hosts), ICT (25 hosts), Faculty (88 hosts), Bursar (7 hosts).

Step 1: Always sort departments in descending order of host count!
1. Admin (150 hosts) $\rightarrow$ Needs $2^8 - 2 = 254$ hosts $\rightarrow$ /25 (Allocates 10.1.1.0 to 10.1.1.127)
2. Faculty (88 hosts) $\rightarrow$ Needs $2^7 - 2 = 126$ hosts $\rightarrow$ /25 (Allocates 10.1.1.128 to 10.1.1.255)
3. Marketing (60 hosts) $\rightarrow$ Needs $2^6 - 2 = 62$ hosts $\rightarrow$ /26
4. ICT (25 hosts) $\rightarrow$ Needs $2^5 - 2 = 30$ hosts $\rightarrow$ /27
5. Bursar (7 hosts) $\rightarrow$ Needs $2^4 - 2 = 14$ hosts $\rightarrow$ /28