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TOPIC: Classful & Classless routing subnet mask question

Classful & Classless routing subnet mask question 9 years 4 months ago #22609

  • shamrock
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Hi there.
I have a question regarding classful and classless routing.
From what I gather when you deploy a classful routing protocol
for your network (Such as RIP V1) all devices must share the
same subnet mask as RIP V1 does not send subnet information
along with it's route updates.

:?: But what Im trying to clarify is that if RIPV1 does not support classless routing and every device in the network must share the same subnet mask (when using RIPV1), must the subnet mask be a classful one?
I.E 255.0.0.0 or 255.255.0.0 or 255.255.255.0
would it route a packet to a destination network with an address such as 172.16.192.0/18 if every device in the network has the same subnet mask - 255.255.192.0 or would it only route if the subnet mask was a default class B subnet mask such as 255.255.0.0??
Many thanks for your help!
Great site by the way!
Rgds,
Shamrock
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Re: Classful & Classless routing subnet mask question 9 years 4 months ago #22613

  • skepticals
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It is my understanding that the subnet mask must be a class A, B, or C. I am not sure what RIP v.1 would do with different subnet masks.
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Re: Classful & Classless routing subnet mask question 9 years 4 months ago #22614

  • S0lo
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Your network should not necessarily use the same subnet mask. But all the subnet masks should be classfull, either /8 /16 or /24

However, for an interface (subnet) that is directly connected to a router and manually configured to use a non-classfull mask, the router will use this subnet mask to build it's routing table even though RIPv1 does not broadcast this mask.

The result of all this is an odd case:

When you use ONLY ONE non-classfull subnet mask for ALL your network. IT WILL WORK correctly even with RIPv1. However, if you use MULTIPLE non-classfull subnet masks, its not guaranteed to work with RIPv1

Thats as far as I know. Experts please correct me if I'm wrong.
Studying CCNP...

Ammar Muqaddas
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Re: Classful & Classless routing subnet mask question 9 years 4 months ago #22615

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Studying CCNP...

Ammar Muqaddas
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Re: Classful & Classless routing subnet mask question 9 years 4 months ago #22633

  • shamrock
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Ok I see.
Thats pretty odd alright.

Doesnt RIPV1 auto summarize networks at their classful boundaries and wouldn't this mean that you simply cannot have any subnets in your network when using RIPV1?

i.e

router(Config)#router rip
router(Config-router)#network 172.16.0.0
router(Config-router)#network 192.168.10.0
router(Config-router)#network 10.0.0.0

If RIPV1 can only populate its routing table with the above info,
any network with a classless mask simply wont matter, it wont make it into the routing table.
Maybe im wrong.
Thanks again for your input!
Regards,
Shamrock
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Re: Classful & Classless routing subnet mask question 9 years 4 months ago #22642

  • toddwoo
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Your pretty much right...with a classfull routing protocol you don't get to use non classfull subnets.
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