O'Reilly logo

CCENT/CCNA ICND1 100-101 Official Cert Guide by Wendell Odom

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

Finding the Subnet ID: Difficult Masks

The following written process lists all the steps for find the subnet ID, using only decimal math. This process adds to the earlier process used with easy masks. For each octet:

Image

Step 1. If the mask octet = 255, copy the decimal IP address.

Step 2. If the mask octet = 0, write a decimal 0.

Step 3. If the mask is neither, refer to this octet as the interesting octet:

A. Calculate the magic number as 256 – mask.

B. Set the subnet ID’s value to the multiple of the magic number that is closest to the IP address without going over.

The process uses two new terms created for this book: magic number and interesting ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required