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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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    Welcome to Network Engineering! For two IP hosts in different networks to communicate, you need to route between them. What you're asking to do can't be done without a lot of "magical" configuration.
    – Ron Trunk
    Commented Nov 29, 2018 at 16:56
  • I beg to differ! See answer below.
    – jonathanjo
    Commented Nov 29, 2018 at 18:25
  • If one of the answers helps you, do click "accept" -- you get a few points and it stops the question popping up as a zombie in a few months time. Or, give you own answer and accept that.
    – jonathanjo
    Commented Nov 30, 2018 at 15:20
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Commented Dec 25, 2018 at 10:05

2 Answers 2

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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.

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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.

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R1:

interface Serial0/0/0
 ip address 131.43.43.1 255.255.255.224
!
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 

R2:

interface Serial0/0/0
 ip address 131.44.44.2 255.255.255.224
 clock rate 2000000
!
interface Serial0/0/1
 ip address 140.10.10.2 255.255.255.224
 clock rate 2000000
!
ip route 131.43.43.0 255.255.255.224 Serial0/0/0 

R3:

interface Serial0/0/1
 ip address 140.10.10.3 255.255.255.224
!
ip route 0.0.0.0 0.0.0.0 140.10.10.2 

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