Pls help me About Subnetting

(Classless Inter-Domain Routing) A method for creating additional addresses on the Internet, which are given to Internet service providers (ISPs) that in turn delegate them to their customers. CIDR reduces the burden on Internet routers by aggregating routes so that one IP address represents thousands of addresses that are serviced by a major backbone provider. All packets sent to any of those addresses are sent to the ISP such as MCI or Sprint. In 1990, there were about 2,000 routes on the Internet. Five years later, there were more than 30,000. Without CIDR, the routers would not have been able to support the increasing number of Internet sites.Instead of the fixed 8, 16 and 24 bits used in the Class A-B-C network IDs, CIDR uses a variable network ID from 13 to 27 bits. For example, the CIDR address 204.12.01.42/24 indicates that the first 24 bits are used for network ID.

source: www.techweb.com/encyclopedia

Techweb is the best source (in my opinion) for easy to understand, yet technically accurate definitions of IT related terms. I use it a lot.

As far as how to subnet, I have some old hand written notes from a couple years ago that I will try to fit into the limited comment space below.

Stay tuned....

Good explanation above...

For practical purposes, you might want to use a subnet calculator, which does all the work for you! There are many free ones available for download.

You just plug in an IP address, how many subnets you need, how many nodes on each subnet and voila! It tells you what your subnet/CIDR mask should be for each subnet.

In my opinion, the best one is from Net3 Group -- I tried to find a link to put here, but can't seem to locate one.

If you e-mail me privately, I'll e-mail you with it (448K). It's free and can be freely distributed.

Tom G

Boson has a great free tool and I use it religiously.

http://www.boson.com/promo/utilities/subnetter/ip_subnetter.htm

Another great free site is from wildpackets;
http://www.wildpackets.com/products/ipsubnetcalculator

Solarwinds also has afree subnet calculator:
http://support.solarwinds.net/updates/New-customerFree.cfm

(by the way, their toolsets rock...)

Cisco tech web site has these VERY detailed technical notes (link may wrap):

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/index.htm

Specifically, the subnetting theory is contained in this chapter on Internet Protocols:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ip.htm

Also, from this page: http://www.cisco.com/warp/public/759/ipj_2-4/ipj_2-4_oneb.html

There is this description of CIDR:

CIDR
Classless Inter-Domain Routing or CIDR (RFCs 1518 and 1519; September 1993) was introduced to improve both routing scalability and address space utilization in the Internet. By eliminating the notion of "network classes," CIDR allows for a better match between address requirements and address allocation. This results in expansion of the scope of hierarchical routing, which in turn improves scaling properties of the Internet routing system. CIDR has proven to be the palliative that has enabled the Internet to continue functioning while growth continues.

This all helped me with my recent quest to learn about subnetting, I hope it helps you also.

Cheers,

Michael Earley
Brisbane, Australia

Tom has given you a bloody good explaination. However, if you are setting up a subnet for a business then you are best to give yourself plenty of expansion and use either the reserved class A (10.x.x.x) or the Class B (172.1.x.x) address ranges. These have the added advantage that if any traffic leaks out to the Internet it will get killed at the first router it reaches.

The IP addresses 10.x.x.x, 172.1.x.x, 192.168.0.x are reserved for internal use only and Internet routers and PCs are set to ignore them.

If your network is connected to the Internet you will have to have a gateway set up to share the connection, thus having it provide Network Address Translation services and using one of these ranges adds security. If it is not connected it does not matter what addresses you use.

I tend to use the 10.x.x.x as it allows me to easily break up sub-nets based on operational and geographical groupings without needing a subnet calculator for most networks. Also give room fro expansion.