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Community Member

Going from 'flat' to subnets..


Presently our network has all of it's connections going to a 2980g switch. We are one large broadcast domain. We want to introduce layer 3 into our design. We have a 32 etherport 8510csr router/switch. We have 20 network segments going to the 2980g that we want to subnet and connect to the 8510csr.

We are all behind a firewall with the entire addressing. We have several servers fix with 10.0.0.x addresses that cannot be changed because we have client software at many workstations pointing at these addresses.

My question is this. What would you recommend or suggest as a subnetting scheme to apply to these etherports? I know I will have to change the class A default subnet mask.. but to what? Class B or C? 254 addresses per subnet would be plenty. Please advise, as we are very nervous about this project and concerned that we get it right the first round. Pleasse supply examples if possible.



Community Member

Re: Going from 'flat' to subnets..

Hello, Gary

First: Are you realy NEED or ONLY want to change design? Do you have any problems with flat design?

Second: You don't need now to follow classfull address scheme. You can use classless one. If you need 1500 real addresses you may use /21 network mask. Use any scope from RFC1918 (,,

If you have to keep addresses and network mask use 172.16 & but i think you can change ONLY network mask at servers, split you network, but don't change pointing addresses at PC, because of addresses of server you'll not to change.

Good luck



Re: Going from 'flat' to subnets..

Thanks Paul for your valuable help. We seem to be fine on a flat network thus far. And yes, I can only change the mask at the servers not the ip addresses. I am just concerned that we are adding segments and more stations all the time., growing our broadcast domain. Our 2980g has only two ports left and we have an 8510csr that is in the router cabinet but not in use.

Do you see any issues with the servers being fixed with 10.0.0.x numbers and the dhcp segments all being 10.0.x.1, 10.0.x.1, ect? Once I have these subnets/segments active, does this mean I need then to be 'classless'.

My thinking is, I bring everything down. Bring up the routers and firewall and change the subnet mask on them. Add the 8510csr to the mix. Reconfigure the dhcp in windows 2k and assign all the 8510csr ports a subnet address and test with a workstation for correct address info. If successful, bring up the servers and change their subnet mask and test again.

I was thinking of just using a class c subnet on the class A addresses and using 254 host ip's per subnet.. this should be more than enough ip's.

Do you have any suggestions or can you think of anything I may have missed. Do you see any additional static routes I would need to add anywhere?



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