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classful routing

HI, this might seem a very basic question but i m a bit confused, i was reading jeff doyle 1 first edition, in chapter 11 under the topic of "Redistributing from classless to classfull Protocols" (FIG 11.12) he has given example ofa router with 4 interfaces, e1 =

e0 =, s 0 =, s 1 =

he said the subnet of s 0 which is of mask 30 is not advertised out e1 and e0 since it cannot use the 27 bit mask to derive 30 bit subnets, and it cannot use 30 bit mask to derive 27-bit subnets, i m a bit confused here why it cant derive the masks ? all i know abt classful is that subnet mask is not send in updates, is that the only rule shall i be concerned abt? or this example is saying something more that i shuld know ?? plz tell me either refer me any doc or plz explain

thanks in advance


Re: classful routing

Here is something about how classfull masks are determined by routers:

- Only a few bits (4) in the first octet(byte) are examined

- 0xxxxxxx -> Class A or /8

- 10xxxxxx -> Class B or /16

- 110xxxxx -> Class C or /24

Straight forward. So there is no need to send masks in classful routing.

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Re: classful routing

Lets review the addresses and masks

e1 =

e0 =

s 0 =

s 1 =

So the router knows that it has subnets and If the router DID advertise the serial subnets to its neighbor on E0 then the neighbor receives and since the neighbor did not receive advertisement of the mask it will assume that its own mask should work (/27). But there are a couple of problems with this. First is not a valid subnet address with a /27 mask. Second (and more serious) the neighbor would believe that was in the subnet of the neighbor but it is not.



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