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Home > Archive > CCNA > February 2004 > why it is wrong - IP subnet?
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why it is wrong - IP subnet?
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| worrywarm 2004-02-22, 3:57 pm |
| PC1-Switch1-Router1-Router2-Switch2-PC2
Gaven the figure above and the IP address scheme, as follows:
PC1:
ip 192.168.20.18/29
GW 192.168.20.17
Router1:
E0 192.168.20.17
S0 192.168.20.49/28
Router2:
E0 192.168.20.97
S0 192.168.20.50/28
PC2:
ip 192.168.20.98/29
GW 192.168.20.97
Question, the scheme does't work, why?
Answer:
the subnet masks are invalid.
My doubt is what it mean of invalid??
As you see in the figure, both E0 do not have a subnet mask following the IP address. Suppose the subnet masks for both E0 are /29, this should be a valid scheme, am I right? The point is the invalidation only lies in no configuration of subnet masks of E0s.
(It doesn't mention what kind of routing protocol it uss, classful or classless)
Anyone can help??? Thanks a lot  | |
| dmaftei 2004-02-22, 10:24 pm |
| The addresses on the routers are invalid. On both routers you have the ethernets in subnet 192.168.20.0/24, and the serials in subnet 192.168.20.48/28. The router(s) won't let you do that. You either put the ethernets in a subnet other than 192.168.20.0/24, or in a 192.168.20.0/28 subnet other than 192.168.20.48/28 (.0/28, or .16/28, or .32/28, or .64/28, etc.)
The missing mask for the ethernets implies they are in the natural class (class C for 192.x.x.x). | |
| worrywarm 2004-02-23, 11:59 am |
| quote:
Originally posted by dmaftei
The addresses on the routers are invalid. On both routers you have the ethernets in subnet 192.168.20.0/24, and the serials in subnet 192.168.20.48/28. The router(s) won't let you do that. You either put the ethernets in a subnet other than 192.168.20.0/24, or in a 192.168.20.0/28 subnet other than 192.168.20.48/28 (.0/28, or .16/28, or .32/28, or .64/28, etc.)
The missing mask for the ethernets implies they are in the natural class (class C for 192.x.x.x).
what if the router uses the classless routing protocol, then we can say that,
ethernet on router1 is in 192.168.20.16/29
ethernet on router1 is in 192.168.20.96/29
Serial ports are in 192.168.20.48/28
It should be valid network supporting classless routing,is it? I remeber I 've seen it somewhere.
If the missing mask implies they are the Class C, this configuration would be invalid because of that instead of ethernets using the same subnet.
????
Thank you very much!!! | |
| dmaftei 2004-02-23, 12:23 pm |
| quote:
Originally posted by worrywarm
what if the router uses the classless routing protocol...
AFAICS this has nothing to do with routing protocols; it's all about the addressing scheme. And if the routers' ethernets were /29, then the scheme would be valid. | |
| edmonds_robert 2004-02-23, 12:57 pm |
| It's a poorly worded question. If you are going to leave out the subnet masks for the Ethernet interfaces, then it should be assumed that they are the same as the network to which they connect. You won't have a question like this on the CCNA, as far as I remember. (WIth missing subnet mask info, I mean) |
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