Need Subnetting simplified 1

[gurner]

I know i could understand subnetting and reading up on it makes sense to a certain extent.

But i want to be able to answer Subnetting questions in an upcoming Win2k NetInfrastructure exam and CCNA exam without claming up or suffering from Brain Fade.

At present i am having difficulties retaining what i am learning on it, EVERYTHING else is fine, just Subnetting goes out the window (or my Ears) as soon as the book is put down.

Does anyone have a Laymans term way of remembering it, something easy to recall?

The OSI model can be memorised by "All People Seem To Need Data Processing" "Application Presentation Session Transport Network Datalink Physical"

 

[manarth]

Nothing simple: but...

IP addresses are normally written as dotted octets; e.g.

IP = 192.168.0.1
SN = 255.255.255.0

This is more memorable than decimal; e.g.

     192     168     0       1 
IP = 11000000101010000000000000000001
SN = 11111111111111111111111100000000
     <-------network--------><-host->

Take a look at the decimal IP/Subnet...
The 1s on the SN indicate that this part of the IP address indicates your local network (this part of the ip is the prefix). A destination IP sharing the same prefix will only be broadcast on the local network; it won't be routed.
If the destination IP has a different prefix, it will be sent to the default gateway for routing.

With above example;
any IPs beginning 192.168.0. should be on the same physical network, so won't be routed.
anything else will.

Subnetting is now classless (CIDR) but references are still made to the old classes....

Class A = subnet 255.0.0.0
Class B = subnet 255.255.0.0
Class C = subnet 255.255.255.0

With CIDR, an IP is given as follows;
IP = 192.168.0.3/24
The first part is the IP address
The /24 indicates that the first 24 bits of the subnet mask are 1; i.e. 11111111111111111111111100000000 - or more commonly 255.255.255.0

Class A subnet is /8
Class B subnet is /16
Class C subnet is /24

Other network ranges can be used with classless addressing; e.g. /14

IP = 192.168.0.1/14

     192     168     0       1 
IP = 11000000101010000000000000000001
SN = 11111111111111000000000000000000
     <--network---><-------host----->

Which indicates any IP starting 11000000101010 is on the same network, so won't be routed - anything else will.

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