Query about a problem : CCNA

maveric121

Suppose that there are 9 hosts connected to a Router Fa0/0 interface through a switch. Fa0/0 interface of router has Ip address 192.168.30.30/30. This interface will be the default gateway of the 9 hosts on the LAN. The Ip address of the 9 hosts fall in the range of 192.168.30.16/28

Now the problem with the network is that the users are not able to acces the internet connected to the S0/0 interface of the router.
I know the reason for this is that the Fa0/0 interface 9default gateway) has a different mask (/30) as compared to the Lan hosts mask (/28 )

What I want to ask is that :
1. Why the mismatch of the mask is causing some of the host not to connect to internet and some of the host to connect to intyernet ?

JavonR

For the hosts to be able to access the default gateway they need to be in the same subnet. By using a /30 mask on the gateway you are effectively putting it in a different subnet then most of your hosts, hence why most cannot communicate. Hope that answers it for you

Dubuku57

Just out of curiosity, are the ones that can comms the hosts with .28 and .29?

Also, did you enable routing and default route to internet?

Neeko

JavonR wrote: »

For the hosts to be able to access the default gateway they need to be in the same subnet. By using a /30 mask on the gateway you are effectively putting it in a different subnet then most of your hosts, hence why most cannot communicate. Hope that answers it for you

I think he is aware of this. The question is why the mismatch affects some hosts but not others.

The hosts that aren't affected should be 192.168.30.29 and 192.168.30.30. This is because the two networks overlap, and these are the IPs that are in that overlap.

When packets return to the router and are destined for either of the addresses I have listed, due to the overlap the router sees these as belonging to the 192.168.30.28 /30 network that Fa0/0 is configured for. Remember the destination and source IP addresses do not include subnet masks, so a router will treat packets based on it's own settings and routing table.

Any of the hosts with IP's that are between 192.168.30.17 and 192.168.30.27 will be dropped because it has no interface configured in a network that contains those IPs.

maveric121

Thanx a lot every one

Thanx Neeko !!

Neeko

Was I right about which hosts it didn't affect?

nanga

if the fa0/0 on the router is the default gateway then it has to be in the same subnet as to what the switch is /28 for the switch to talk to the router