R1 and R2 are connected with serial cable interface but both of them having different subnet and i know they will not communicate so what are other options two communicate between them without the concept of multiple ip's
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You can configure them with point-to-point routes as follows, as tested in Packet Tracer (three 2901 routers with HWIC-2T). R1 has default route to interface, R3 has default route to next-hop address, and R2 has a manual route to interface plus an interface route.
interface Serial0/0/0 ip address 126.96.36.199 255.255.255.224 ! ip route 0.0.0.0 0.0.0.0 Serial0/0/0
interface Serial0/0/0 ip address 188.8.131.52 255.255.255.224 clock rate 2000000 ! interface Serial0/0/1 ip address 184.108.40.206 255.255.255.224 clock rate 2000000 ! ip route 220.127.116.11 255.255.255.224 Serial0/0/0
interface Serial0/0/1 ip address 18.104.22.168 255.255.255.224 ! ip route 0.0.0.0 0.0.0.0 22.214.171.124
A serial interface is (often) an "unnumbered" interface, ie. there's no need for an IP address binding (nor does one usually matter).
Ethernet interfaces and the like require an address because Ethernet's data link layer doesn't work without a layer-2 address, and a layer-3 address is required to find out a router's local layer-2 address. A serial interface simply transmits a packet from source to destination without any need of addressing; it's always point-to-point.
What you do need though are routing table entries for the remote subnets as jonathanjo has already pointed out. These can be set up statically or exchanged via a routing protocol like OSPF.