General Subnetting Rules

[rbeckwith]

Hello,

I am looking for a solid resource on subnetting do's & don'ts. Can someone please point me in the right direction.

Thanks

Rod

[Prashanth Krishnappa]

The following white paper is a good resource

http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf

Also, take a look at the following pages

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd20a.htm

http://www.cisco.com/warp/public/701/3.html

[rbeckwith]

Thanks for the pointers.

I guess I should be more specific. I recall a rule about subnets be contiguous.

I have someone who has a class C network that has a .255 mask which gives them 254 addresses.

They them decided to partially subnet the adresses by keeping the .255 for a certain portion of network & then assigning a .240 to a router one hop away.

This shouldn't work from what I remember:

Here is the example

Router A

209.17.XXX.1 255.255.255.0

|

V

Dumb Ethernet Hub

|

V

Router B

Int e 1

209.17.XXX.129 255.255.255.0

|

V

MPP Serial Link(2 T-1's)

192.168.1.1 255.255.255.0

|

V

Router C

MPP Serial Link(2 T-1's)

192.168.1.2 255.255.255.0

|

V

Int e 1

209.17.XXX.145 255.255.255.240

Does this make sense?

Rod

[efrahim]

This will not work.

Erum

[rbeckwith]

Erum,

Thanks, I didn't think it would....is there a good document that explains why this doesn't work or could you so I can have a record of it. Is it in the RFC?

Is there a way to make this work if the main network was subnetted from the

beginning like the following?

Here is the example

Router A

209.17.XXX.1 255.255.255.240

|

V

Dumb Ethernet Hub

|

V

Router B

Int e 1

209.17.XXX.129 255.255.255.240

|

V

MPP Serial Link(2 T-1's)

192.168.1.1 255.255.255.0

|

V

Router C

MPP Serial Link(2 T-1's)

192.168.1.2 255.255.255.0

|

V

Int e 1

209.17.XXX.145 255.255.255.240

or would it have to be like this?

Router A

209.17.XXX.1 255.255.255.240

|

V

Dumb Ethernet Hub

|

V

Router B

Int e 1

209.17.XXX.129 255.255.255.240

|

V

MPP Serial Link(2 T-1's)

209.17.XXX.163 255.255.255.240

|

V

Router C

MPP Serial Link(2 T-1's)

209.17.XXX.161 255.255.255.240

|

V

Int e 1

209.17.XXX.145 255.255.255.240

Thanks

Rod

[efrahim]

no that will also not work. .. Here are some good URL

http://www.cisco.com/warp/public/701/3.html

http://www.cisco.com/warp/public/701/8.html

You can't use the same network if you are going to connect the network like this . Subnetting will allow to divide the one big Class a,b or c networks to small subnets means small network..

like the mask 255.255.255.240 will be allow you to have the 14 host -

Assume XXX-1

like 219.17.1.1--- 219.17.1.14 is valid IP address for one subnet (network -219.17.1.0), then 219.17.1.17-219.17.1.30 for second range( 219.17.1.16). and so on there no overlapping ip address

then you networks are overlapping and that 'll not work.

In you configuration if I assume XXX=same number like 1

for first two routers - you can have 219.17.1.1 and router B interface should be 219.17.1.14 and for the end routers -- 219.17.1.17-- 219.17.1.30 with mask 255.255.255.240 for all the top and bottom routers. and if you want to use differnent subnets mask for more host, make sure you don't have overlapping ips.

Please use the subnet calculator.

http://www.cisco.com/cgi-bin/Support/IpSubnet/home.pl