Secondary IP addressing doubt...

hi all

i was studying for my CCNA-ICND exam and i was looking at the Secondary IP addressing concept. Its fairly easy to understand where u can assign a secondary ip subnet on a router and hv a bigger pool of addresses BUT what i fail to understand is that i learned in INTRO that 2 subnets HAD to be seperated by an IP router. doesnt this go against this rule?....any help would be great.

Thanks in advance!

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Webmaster

No it doesn't, as traffic that needs to be routed from one subnet to another will go thru the routing process, even if they are connected to the same physical interface.

amod_pandya

but what do they really mean when they say "different subnets need to be seperated by a router" ?...im a little confused here.

suppose 10.1.1.2 needs to send data to 10.1.1.3..that would go thru the router AND SO WOULD data to 10.1.3.1 (secondary subnet) ..so what does the earlier statement really mean?

Thanks,

Webmaster

They mean that if you connect two subnet and want to allow communication between hosts from both subnets, you need to connect them with a router.

That is why I mentioned routing process. If both subnets are connected to the same physical interface, using secondary interfaces, and a host in one subnet wants to communicate with a host in the other subnet, the data will be routed by the router's routing process. I.o.w. traffic will go into the physical interface into the router and be routed out the same physical interface.

suppose 10.1.1.2 needs to send data to 10.1.1.3..that would go thru the router

That would not go thru the router. As both are on the same subnet (I assume you use a 255.255.255.0 mask) they can communicate directly (the source host will use ARP to resolve the IP address of the destination to a MAC address and start sending frames directly.) If 10.1.1.2/24 wants to communicate with 10.1.3.1/24 the source host will send traffic to its default gateway (interface on the router) which will forward it out the same physical interface but secondary.

I hope this helps,

Johan

amod_pandya

Yes i think it makes sense. So what you are saying is that data from 10.1.1.2 to 10.1.1.3 (same subnet) would not undergo a "routing process".....

....But the data would go thru the router wouldnt it?...assuming that there is no switch or any other device on the LAN and ALL the PC's are connected DIRECTLY to the router, something like a home-network. All Im trying to say is that router would serve as an intermediate medium when the data goes from 10.1.1.2 to 10.1.1.3 but wont do any routin table lookup. Is this true?

Forgive me if i sound like a newbie but thats cos i AM!! i really have no practical experience in networking just playing around with my internet router in my apartment is all i knew before studying for CCNA.

THANKS A TON!

Webmaster

So what you are saying is that data from 10.1.1.2 to 10.1.1.3 (same subnet) would not undergo a "routing process".....

Indeed.

....But the data would go thru the router wouldnt it?...assuming that there is no switch or any other device on the LAN and ALL the PC's are connected DIRECTLY to the router, something like a home-network.

Well there is a switch or hub

Those routers used in home networks, ie. a cable or ADSL access router, is usually a router with integrated switch ports. If you connect a single host to each physical interface on a cisco router, they wouldn't be in the same subnet, and you would not need secondary interfaces.

amod_pandya

yes now its finally clear to me....all this while i thought so differently of those home internet routers....!!

thanks so much man!

Webmaster

You're most welcome