Inside an Autonomous System, it is possible to move a machine inside a network, keeping its IP address even though it goes to a network segment that doesn’t serve the corresponding Network.
Something like this:
RouterA -------- Network Segment | Host A (10.1.0.2/24, GW: 10.1.0.1)
It is possible to move Host A to another network segment, lets say to interface FastEthernet of RouterB. Of course this would require address and possibly other configuration changes to Host A. Changin the IP address of a server is not always a good idea ™.
Lets say that we move Host A to interface FastEthernet of Router B. Supposing that a routing protocol is setup and works in the Autonomous System, Host A may keep its IP address by configuring RouterB (cisco commands):
RouterB(config)# ip route 10.1.0.2 255.255.255.255 FastEthernet 0/1 RouterB(config)# interface FastEthernet 0/1 RouterB(config-if)# ip proxy-arp # This is the default RouterB(config)# router eigrp 1 RouterB(config-router)# redistribute static RouterA(config)# interface FastEthernet 0/1 RouterA(config-if)# ip proxy-arp
This will work because:
- Whenever Host A tries to reach a host in its subnet (ARP request), Router B will respond with its mac address. This is what Proxy ARP does.
- All routers within the A.S. will learn the 10.1.0.2/32 route. Even Router A will prefer to use this one instead of the directly connected 10.1.0.0/24 since it has a longest preffix (routing table lookups are longest matching preffix lookups)
- Since Router A will also learn this route and since it has Proxy ARP enabled, it will respond to ARP requests for 10.1.0.1 with its address