Supernetting

[jszei]

We have an internal network 192.168.1.0 255.255.255.0 established. We would like to expand it to include 192.168.0.0, 192.168.2.0 and 192.168.3.0.
Can we do this without the use of routers by just changing the subnet mask on all our hosts to 255.255.252.0?

Thanks,
John

 

[jeter]

why not change to a 172.16.1.1 network with a subnet of 255.255.0.0 ? The other way you mention is possible but I would not do it. this is why they came up with

10.x.x.x 255.0.0.0
172.16.x.x 255.255.0.0
192.168.x.x 255.255.255.0



“Reserve your right to think, for even to think wrongly is better than not to think at all”

Fisher CCNA, Network+

 

[jszei]

To change over to the 172.16.x.x class B type network would require changing too many static ips.
Why wouldn't you do it? What problems do you foresee?
You said it is possible, if I were to do it would changing the mask to 255.255.252.0 an all hosts be the only necessary change?

Thanks

 

[manarth]

jszei - what you're describing is using CIDR (classless IP addressing).

not all routers can support CIDR setups, so jeter's suggestion of using a 172.16.x.x / 255.255.0.0 subnet is an easy and safe bet.

<marc> i wonder what will happen if i press this...[ul][li]please give feedback on what works / what doesn't[/li][li]need some help? how to get a better answer: faq581-3339[/li][/ul]

 

[jszei]

We do not have any routers involved in our internal network. Would we need routers to communicate between say, 192.168.1.0 & 192.168.2.0 or would changing the mask on the hosts to 255.255.252.0 work as well?

Thanks

 

[Comstocb]

Kind of a curious question I don't know the answer to...What current routers don't support CIDR? How old will a router be before you need to understand if it supports CIDR or not? I've only been at this a few years and I have never had to worry about a router not supporting CIDR. Maybe I am lucky but with that said, I would run with your plan you outlined jszei with my only concern being you now have just over a 1000 nodes on the same broadcast domain. If your switched, I wouldn't worry too much but shared, its a big problem. Just my close to useless thoughts on the subject.

Brian

 

[jszei]

NB: All our hosts (about 80) are plugged into a single switch.

Thanks

 

[manarth]

jszei - if it's purely an internal LAN - no routers - then you should have no problem supernetting as you've described.

how are the addresses allocated? DHCP or static IPs?

If you supernet you won't need routers between the 192.168.1.0 and 192.168.2.0 - routers are only needed to communicate to a different subnet (as you're supernetting, these IP ranges will be on the same subnet).

with 80 hosts, these could all be on 1 class C subnet; e.g. IP 192.168.0.x and subnet mask 255.255.255.0 (allows 254 hosts)

as you're switched, the number of hosts sharing a broadcast domain will not be a problem.

comstoch - most routers within the last few years will be fine with CIDR; non-compatible routers don't necessarily exhibit symptoms even when in a CIDR environment - it basically depends on the router and the IP ranges allocated.

<marc> i wonder what will happen if i press this...[ul][li]please give feedback on what works / what doesn't[/li][li]need some help? how to get a better answer: faq581-3339[/li][/ul]

 

[jszei]

Thanks manarth, We have some address space reserved for DHCP. Most of the addresses in our 192.168.1.0 network have been used & we're looking to expand. Our DMZ is on our 192.168.3.0 network, some vpn clients get assigned a 192.168.2.0 range. Currently for people to communicate with the dmz or the folks on vpn they need a route statement added to their PC. We're hoping that supernetting will relieve the burden of adding route statements as well. Does anyone know how the master browser handles a supernet in a W2K domain?

 

[Comstocb]

If you supernet the network to include the DMZ, then you no longer have a DMZ as all devices are on the same network meaning any security you were counting on by being in the DMZ are now gone. This just needs to be considered. Also, if your running DHCP (it will work for static equipment as well) and have router interfaces on the 3 networks described (default gateways) then why are your clients not configured with these default gateways eliminating your need to have to configure individual routes to each of these networks?

Brian

 

[jszei]

Our DMZ is on the another interface on a PIX. I agree that supernetting what is on the DMZ would open up that network to the LAN. It would have to get through the PIX first though. We have no routers on our network. If we want a device to communicate with another network we currently have to add a permanent route statement.
So, in light of this info do you see any problems with supernetting? I changed the subnet mask to 255.255.252.0 for a few clients & a server. The problem was none of them appeared in My Network Places. Thus, prompting my earlier question about browse masters. Has anyone done this before?

Thanks,

John

 

[opusx2]

We recently changed a single class C subnet 192.168.1.0 255.255.255.0 to a supernet with 3 class C's just like yours, 192.168.1.0-192.168.3.255 with a subnet mask of 255.255.252.0.

We had to change all our internal servers' subnet mask to reflect the supernet and redefine the DHCP scope in NT. The gateway on our router had to be changed also to reflect the new supernet.