Subnetting - jibingl/CCNA-CCNP GitHub Wiki

Recognization of Given Network ID/Address

One of the trickiest aspects of subnetting is identifying the real network ID/address (i.e., the first IP address of a given network). Sometimes, the starting point of the questioned network is not explicitly given. Mistaking the network address results in all further subnetting calculations incorrect.

Below, we’ll guide you through the process of mathmetically identifying the real network ID using step-by-step examples.

Example 1: Network 192.168.100.115/27

Understanding CIDR/Network Mask
The network mask is /27, which provides the following information:
- Network portion: 27 bits
- Host portion: 32 - 27 = 5 bits
- Total IP Addresses: 2⁵ = 32 IPs
  Breakdown of the Total IPs in a special octet-format:
```
2^5 = 2^0 * 2^0 * 2^0 * 2^5 = 1 * 1 * 1 * 32 --> 1 . 1 . 1 . 32
```

Calculating the Network ID.
To find the network ID, apply a modulus operation on the given network with the octec-format Host portion.
Then, subtract the modulus result from the given network.

      192.168.100.115                  192.168.100.115
 mod)   1.  1.  1. 32              -)    0.  0.  0. 19
 ---------------------    --->     -------------------- 
        0.  0.  0. 19                  192.168.100. 64

The Given Network's ID is 192.168.100.64/27 indeed.

Example 2: Network 10.10.10.10/14

Understanding CIDR/Network Mask
For the network mask /14, we have:
- Network portion: 14 bits
- Host portion: 32 - 14 = 18 bits
- Octet-format Total IPs: 2¹⁸ = 2⁰ * 2² * 2⁸ * 2⁸ --> 1.4.256.256

Calculating the Network ID
Apply the modulus and subtraction operations on the given network:

       10. 10. 10. 10                   10. 10. 10. 10
 mod)   1.  4.256.256              -)    0.  2. 10. 10
 ---------------------    --->     -------------------- 
        0.  2. 10. 10                   10.  8.  0.  0

Network ID: 10.8.0.0/14

Key Formulas or Rules

Host portion: h = 32 - n (where h, n are the number of bits in the Host portion and Network portion respectively)
Octect-format Total IPs: 2^h1.2^h2.2^h3.2^h4 (where h1, h2, h3, h4 are the numbers of host-portion bits available in each octect; h=h1+h2+h3+h4)