Lesson 17: Advanced Subnetting: Middle Octets

When working with Class A or B addresses, the custom subnetting often occurs in the second or third octet (the 'middle octets').

Example: Subnetting 172.16.0.0/16 to /20

• Starting point: 172.16.0.0 with mask 255.255.0.0.
• Target: /20 (meaning 20 network bits).
• We borrow 4 bits (20 - 16 = 4) from the third octet.
• The custom mask is 255.255.11110000.00000000, which is 255.255.240.0.

1. Subnets Created (N=4): $2^4 = 16$ subnets.
2. Block Size (Magic Number): $256 - 240 = 16$. The third octet increments by 16.

Finding Usable Range

In the second subnet (172.16.16.0/20), the broadcast address is the address immediately preceding the next subnet ID (172.16.32.0). Thus, the broadcast is 172.16.31.255.

• First Usable Host: 172.16.16.1
• Last Usable Host: 172.16.31.254