Server move / proxy ARP / NAT?


We have the requirement to relocate a vital server to another subnet.

Unfortunately we have several hundred hosts hard-coded to connect to a fix IP address. Currently clients are not required to route traffic via the default gateway - as the server resides on the same subnet.

Unfortunately this will change when the server is moved behind a router.

Could someone point me in the right direction? Is NAT required? Proxy ARP or some other method that can be used on Cisco routers (Catalyst


Basically (I think) the router will need to "spoof" or proxy traffic heading to server and redirect it to the new address - on the new subnet.

Any suggestions will be appreciated.


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Can the server not be multi-homed with 2 ip addresses - 1 on old subnet, 1 on new.

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Proxy arp and clever routing are only part of the answer.

I can think of at least two approaches.

Approach 1...

Leave both old and new IP addresses on the server. Narrow the netmask on the old IP address as much as possible and create static routes on the server so that server-originated traffic addressed to its old subnet will be routed appropriately through a gateway on its new segment. The ideal situation would be a netmask, but some operating systems may object to this.

On the router connected to the old segment create a static ARP entry pointing toward the router:

arp ARPA alias

On the router (and on any other routers on the forwarding path toward the web server) make sure that there is a host route toward the server's old IP address

ip route ! At last hop ip route ! At intermediate hops

Approach 2...

Get rid of the old IP address on the server.

Deploy the arp alias as before to ensure that traffic is attracted to the router:

arp ARPA alias

On the router, create a host route for the old server IP directing traffic out the appropriate next-hop interface. (I believe that you need to select the right output interface before NAT will reliably kick in).

ip route ! If adjacent ip route ! If not

On the router, create a destination NAT rule.

[I don't trust myself to get the NAT syntax right, sorry]
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Many thanks for the response. It seems "Approach 2" could be a winner!

Using the "arpa alias" command along with the "host route" as you suggested is properly attracting traffic to the router and redirecting to the new subnet. Unfortunately we still have the NAT requirement as the server is now on a new address.

What I can't get my head around is how to change the destination address of packets - as they traverse from the old subnet to the new. My only dealing of NAT has been inside -> outside, where the source address is changed on the way out.

I am guessing some kind of "ip nat outside" command is required but I'm not sure. If anyone can suggest the NAT syntax it would be very appreciated.

So just to clarify...

The server has physically moved networks/subnets. Old address:, now on behind a router. Several hundred clients hard-coded to connect to Need to redirect requests to

Many thanks,


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