I *finally* understand subnetting..... well, most

FLEOHB

I have spent the last month and a half (4 days a week)trying to understand subnetting. After studying today I finally got most of it. The questions that are;

What is the last valid host on the subnetwork 172.29.106.64 255.255.255.192?

And questions like it I understand, what I am having a hard time with is questions like;

How many subnets and hosts per subnet can you get from the network 172.30.0.0/23?

Is there a easy way to work those questions out or do I just need to start memorizing a lot of numbers?

*all example questions taken from subnettingquestions.com

GeeGeez0rz

Ok, just had a look at this one, so.. (im bad at explaining, but ill try)

Its a class B address and the "subnetting" is happening in the 3rd octet. The number of bits being taken from the hosts and added to the network portion is 7. So 2^7 = 128

With the hosts its 2^9-2 (as we now only have 9 bits for the hosts and we take away two for broadcast and network) so that leaves us 510 hosts per subnet.

172.30.0.0
255.255.254.0

2^7 = 128 subnets
2^9-2 = 510 hosts

The way i remember the powers of 2 is i start from 256, i know 2^8 = 256. So all i need to do is remember that one, and I can work up or down. Saves memorising a ton of numbers.

I hope this helps, but im sure someone will be able to explain much better.

fadhil

since the ip address given with CIDR notation is in class B and already subnetting.

In finding number of subnets or hosts per subnet we should work with subnet mask.

since it is class B, it seems that 7 bits have been added from default subnet of class B,i.e 255.255.0.0

we use 255.255.254.0 to compute number of subnets and number of hosts.
hence to get number of subnet just take added subnet i.e 7 bits

2^7 =128 number of subnets.

to get number of hosts just count number of zeros which are 9 zeros

2^9 -2=510 number hosts per subnet.

RomBUS

Wow this link you gave me actually just gave me an awesome memory refresher! I've actually gotten better in a short amount of time. I used to actually know subnetting pretty well but drifted away from Cisco the more I got into the Windows world so I kinda wanna get back (at least ICND1 for starters). Thanks for the link!!