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"Routing" within the same subnet

Our network is composed of a Cisco 2610 router which connects to our ISP and the other end (inside) connects to a switch and then from the switch we connect to a Cisco ASA 5510. Also from the switch we connect to a couple of servers that have public IPs. (Please see attached diagrame with fake IP addresses)

On the router we also have an ethernet port configured for network. This network uses NAT to allow users to connect to internet and that's all we use it for. The IP address: on the outside interface of the router is used for NAT (overload).

As you can see we two different subnets, one from the ISP to our router and the inside public subnet which we use for our servers.

For some reason, the ISP wants to give use only one subnet (outside and inside of the router will be on the same subnet).

How can I configure the router to "route" from outside to inside if both sides are in the same subnet? (I call it "route" because routing means there are two different subnets.

If it is possible please send me some link to some info where I can study this further.

I would appreciate any suggestion.

Thank you,


Hall of Fame Super Silver

Re: "Routing" within the same subnet

Hello Gentian,

you cannot route in the same subnet.

You can use a different type of NAT for the two servers so that they can be reached from internet you can use nat static to map to a specific tcp port of the public address each server.

This can work if the servers are using different TCP ports for their services: example web server uses TCP 80 , smtp mail TCP 25.

If this is the case you can define two more specific nat rules that will map at the socket level: tcp 80 on public addr tcp 80 tcp 25 on public addr tcp 25

ip nat static source tcp 80 tcp 80

ip nat static source tcp 25 tcp 25

and the subnet for the two servers will become private with ip nat inside.

Hope to help


Community Member

Re: "Routing" within the same subnet

I am aware of not being able to route into the same subnet that's why I call it "route" in parenthesis.

The question still remains: how can I use the router if I have the same subnet in two different interfaces.

I need to convert the router into a kind of switch.

Hall of Fame Super Silver

Re: "Routing" within the same subnet

Hello Gentian,

forgive my correction it was clear you were aware of not being able to route within a subnet.

NAT is a possible choice as a I tried to explain in my post.

I think it is difficult to use IRB Integrated routing and bridging because I'm afraid your provider is leaving you only the WAN subnet: so or that subnet isn't simply a /30 so you can do some form of bridging or you need NAT.


bridge 1 protocol ieee

bridge 1 route ip

int bvi1

ip address

ip nat outside

int fas0/0

no ip addr

bridge-group 1

desc to isp

int fas0/1

no ip addr

bridge-group 1

desc to servers

Hope to help


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