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New Member

config assistance: splitting subnet into two

We want to take an ethernet subnet, say a /20, and split into separate

ethernet subnets (probably vlanned) for the purpose of further subnetting a range of

addresses in a diverse customer environment...

e.g. we want to do something like (vlan not part of this)

interface fe0/0

ip address

interface fe1/0

ip address

interface fe1/1

ip address


Obviously if you try this you will get an overlap error and IOS will not allow it.

Without a point to point interface for the /20, how can we do this?

New Member

Re: config assistance: splitting subnet into two

Hi, I read your posting and I do hope this will help you out. If you use the IP address from your example /20 you have 4096 usable subnets and 4094 usable host per subnet, to me that is alot of hosts and unless you have a hugh network you will not need them all. Looks to me you want to use a /20 subnet mask on fa0/0 and a /24 on fa1/0 and fa1/1 do some research into IP classless command. Check out the link below I think it will help.

auto-summary is a similar concept, but is used for routing, where the

routing protocol would "auto-summarize" the routes using the above obsolete

mechanism. You should also enable "no auto-summary" as well

From Cisco

IP Classless

Where the ip classless configuration command falls within the routing and forwarding processes is often confusing. In reality, IP classless only affects the operation of the forwarding processes in IOS; it doesn't affect the way the routing table is built. If IP classless isn't configured (using the no ip classless command), the router won't forward packets to supernets. As an example, let's again place three routes in the routing table and route packets through the router.

Note: If the supernet or default route is learned via IS-IS or OSPF, the no ip classless configuration command is ignored. In this case, packet switching behavior works as though ip classless were configured.

router# show ip route

.... is variably subnetted, 2 subnets, 2 masks

D [90/4879540] via

D [90/25789217] via

S* [1/0] via


Re: config assistance: splitting subnet into two

Hi Prabhdeep,

I didn't get why do you want to do this and moreover it wont work. Why because means - the router forwards the traffic of subnets 81.91.80-95.x to interface fa0/0.

81.91.82.x subnet traffic has two outgoing interfaces one is fa0/0 and fa0/1, this leads to problem.

I hope u understand what I've said.