subnetting

charunanda

dear sirs,
I am new to networking. now i am doing my ccna. i have a doubt regarding subnetting. my doubt is "why do we need subnetting? why cant we communicate with two pc's at different networks/subnetworks (i.e. one in 192.168.2.1 and the other in 192.168.3.1). at the same time we can communicate if the two computers are on the same network." for this communication we need routers. but my question is why? i havent got a best answer yet. so dear sirs please give me an answer which will erase my doubt.
thanking you in advance.

with regards and respect

charu:)

Todd Burrell

The main reason is to break up broadcast domains. If you had one big network you would have all of the machines in one network and you would get severe performance hits from all of the broadcasts. Routers separate networks and keep the broadcast domains smaller so there is not as large of a traffic issue - this is due to the fact that routers do not route broadcast traffic.

andy4tech

You need to try to understand what subnetting really is,well as per my own definition subnetting is a way breaking down a larger network into a smaller ones.Also as per the other question, computer on two different networks will not communicate unless there is a router between them ,routers are used to stop broadcast and to route information from one network to the other.

charunanda

thanks sir. i know subnetting is user for breaking down large network into small manageable subnetworks. to reduce the collision domin and improve trafficking between the network. but my doubt remains why we cant communicate between the two specific hosts in different networks(for example one in class b and one in class c, or like) without using routers?

charunanda

thanks sir. i know subnetting is user for breaking down large network into small manageable subnetworks. to reduce the collision domin and improve trafficking between the network. but my doubt remains why we cant communicate between the two specific hosts in different networks(for example one in class b and one in class c, or like) without using routers?

hiddenknight821

charunanda wrote: »

thanks sir. i know subnetting is user for breaking down large network into small manageable subnetworks. to reduce the collision domin and improve trafficking between the network. but my doubt remains why we cant communicate between the two specific hosts in different networks(for example one in class b and one in class c, or like) without using routers?

What?! You got me baffled. I guess "broadcast" is not a good enough answer for you. If you really want to do us including yourself a favor, then please don't bother asking. Because what you are asking is damn-near impossible. If you really want to know the answer, then you would have to read RFCs and conduct an experiment to determine why your hypothesis can't work.

By the way, we can say your solution will work without a "router", but you would need a layer-3 switch. However, we know it's technically a router with switching functionality.

instant000

charunanda wrote: »

thanks sir. i know subnetting is user for breaking down large network into small manageable subnetworks. to reduce the collision domin and improve trafficking between the network. but my doubt remains why we cant communicate between the two specific hosts in different networks(for example one in class b and one in class c, or like) without using routers?

You can't, because the devices can't *see* each other on their own, as the point of addressing is to identify which networks hosts are on. Now, you can introduce some things to the equation, so that they can be *forced* to *see* each other, but on their own, this doesn't really happen.

One of the best ways to illustrate this, is if you look over what I post below, and then try the exercises that I post below it, to gain a little exposure/depth.


Look at what happens, when I send a "broadcast" to the other host. It will attempt to reply, BUT IT CANNOT FIND ME, as it doesn't have a way to get to my network.

Network diagram:

<[[[r5]]][intf0/0][192.168.6.1/24]>====<[192.168.7.1/24][intf1/0][[[r6]]]>

r5#sh ip int brief
Interface                  IP-Address      OK? Method Status               Protocol
FastEthernet0/0            192.168.6.1     YES manual up                    up
FastEthernet1/0            unassigned      YES unset  administratively down down
FastEthernet2/0            unassigned      YES unset  administratively down down                                                          
r5#debug ip packet
IP packet debugging is on
r5#ping
Protocol [ip]:
Target IP address: 255.255.255.255
Repeat count [5]: 1
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 255.255.255.255, timeout is 2 seconds:

*Mar  1 00:42:16.719: IP: s=192.168.6.1 (local), d=255.255.255.255 (FastEthernet0/0), len 100, sending broad/multicast.
r5#

r6#sh ip int brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES unset  administratively down down
FastEthernet1/0            192.168.7.1     YES manual up                    up
FastEthernet2/0            unassigned      YES unset  administratively down down
FastEthernet3/0            unassigned      YES unset  administratively down down
r6#debug ip packet
IP packet debugging is on
r6#
*Mar  1 00:42:10.155: IP: s=192.168.6.1 (FastEthernet1/0), d=255.255.255.255, len 100, rcvd 2
*Mar  1 00:42:10.159: ICMP: echo reply sent, src 192.168.7.1, dst 192.168.6.1
*Mar  1 00:42:10.163: IP: s=192.168.7.1 (local), d=192.168.6.1, len 100, unroutable

The following are some things you can attempt, to play with this a bit more, and gives you learning opportunity to explore the RIB, versus the FIB, and see how you can make the router give you a response.

1. turn off cef, then ping
2. turn on static routes, then ping
3. turn on cef, then ping
4. turn off static routes (leaving cef on) then ping
5. toggle cef/routing on one host or the other, in different combinations, and see the results.

cyberguypr

Let me give it a shot. I'll use the United States Postal Service example based on the one that Wendell Odom uses in his book. Zip codes are specific to an area. One postal office knows everything within the zip code it serves, but knows nothing about other zip codes. In order to send letters or parcels to other zip codes they have to route them through something that knows about other zip codes. In this case it would be a mail sorting facility who is aware of other zip codes. Without this, the local postal office would only be able to deliver mail to your local community.

Going back to the IT world, by definition networks are isolated and selfish; they only talk to themselves. The router knows general information about about other isolated and contained networks that wouldn't talk to each other by themselves. He is responsible for acting as a gateway between those networks.

It would be like you speaking German and me speaking Spanish exclusively. No matter what we do we will never be able to understand what each other is saying. We would need to route our communication through an interpreter to have our messages delivered and understood.

instant000

Oh wait, I should have given a shorter answer.

You need routers to get between networks, as that is what the function of routers is.

Also, read this document. Where they say "gateway" just insert the term "router"

http://www.ietf.org/rfc/rfc791.txt

The term gateway actually can mean other than a router, but in the link given, it's clearly referring to a router function.

cyberguypr

instant000 wrote: »

Oh wait, I should have given a shorter answer.

Good idea.

instant000

cyberguypr wrote: »

Good idea.

You can lead a horse to water, LOL.

hiddenknight821

instant000 wrote: »

You can lead a horse to water, LOL.

That was more simpler than the post you posted earlier.

charunanda

thank you sirs for your kind and insightful replies. this one doubt was there for a long time in my mind and i asked the tutor about this. but his reply was not convincing me. so i was in search of a platform where i can find a convincing reply. today i got it from you. once again thanks for everyone.

instant000

charunanda wrote: »

thank you sirs for your kind and insightful replies. this one doubt was there for a long time in my mind and i asked the tutor about this. but his reply was not convincing me. so i was in search of a platform where i can find a convincing reply. today i got it from you. once again thanks for everyone.

ASP, Another satisfied poster!